I am measuring from the power pins ...
You only get the 3V3 when the board is connected to a USB lead as this comes from the serial to USB bridge IC and not from external power Vin.
Is yours connected like this?
yes, it is connected to a usb cable and an external power source (9V, 200mA)
I was just wondering if i needed to enable the pins, because the board works, but i am not getting any voltage (3.3v or 5V) out.
It could be that you have a faulty USB lead and you are not actually powering the board from USB.
Have you tried disconnecting the external PSU and just using the USB. There is nothing to enable to get the 3V3 working it is an output from the FT232RL.
In the data sheet it says:-
+3.3V output from integrated LDO regulator. This pin should be decoupled to
ground using a 100nF capacitor. The main use of this pin is to provide the internal
+3.3V supply to the USB transceiver cell and the internal 1.5kJ pull up resistor on
USBDP. Up to 50mA can be drawn from this pin to power external logic if
required. This pin can also be used to supply the VCCIO pin.
I tried powering the board with only the USB cable, and it works (upload and run blink example), but no 3.3V can be measured from the pin.
BTW, Grumpy_Mike, thank you for your help.
Ok my only other thought is that it is broken, however as the USB / Serial chip still works that shows it is not.
Get a schematic of the mega and see if you can measure 3v3 on the capacitor where it comes out of the chip.
The 3.3vdc comes from the FTDI chip's internal voltage regulator. The 3.3vdc pin voltage is available no matter how the board is powered as the FTDI chip is powered by the boards +5vdc buss (after the auto voltage selection circuit) not directly from the USB's +5vdc source.
The most common failure with the 3,3vdc output is if too much current is drawn from it accidently. The FTDI spec is something like 40ma max which is not much, so it's easy to over draw from it and possibly damage it.
As Mike said, you can try and measure for 3.3vdc right at the positive terminal of the C3 capacitor.
Here is the Arduino schematic of the Mega board:
Lefty
The auto power selector is making the power come from the external source instead of the USB port, I guess that is why you are not getting 3.3V... try unplugging the external source.
The auto power selector is making the power come from the external source instead of the USB port, I guess that is why you are not getting 3.3V... try unplugging the external source.
That diagnosis makes no sense?
The board functions, loads sketches, etc, regardless of voltage source, so the boards +5vdc is functioning.
On the Arduino's mega board (unlike the Seeeduino's mega board) the FTDI chip is powered from the +5vdc from after the automatic voltage selection switch. The FTDI chip is always powered regardless of automatic voltage source switch. The Seeeduino Mega is different design, it only powers the FTDI when the USB cable is plugged in and it uses an 0n-board component 3.3vdc regulator to generate it's 3.3vdc power (which also has a much higher current capacity).
The FTDI chip functions, but there in no +3.3vdc, therefore the problem is isolated to the FTDI's internal 3.3vdc regulator or a bad shorted cap or open trace to the +3.3vdc connector pin.
Lefty
i am not getting any voltage (3.3v or 5V) out.
He/She says they are not getting 5v either. Seems very strange.
M T, exactly how and where are you attempting to measure the voltage?
He/She says they are not getting 5v either. Seems very strange.
I missed that one. Hard for the board to function as stated and still not have +5vdc. Maybe a measurement problem with the meter or it's application?
Lefty
Well done digger! 
It's like that scene in Mash when every one is looking one way and the helicopter arrives from the other direction and then they all pretend they weren't actually looking for the helicopter.
Of course if there is not 5V being measured then it is the measuring instrument that is at fault.
Hey, that's rude, you can't always blame the equipment! More often than not, it's user error! 
Do you have the "prongs" in the right slot on your meter? Do you have the right mode selected?:X
Also be sure you are measuring between 3.3v and a common ground.
i.e. if the 3.3v is being feed to a different project, powered separately, the 2 must share a common ground.
Hey, that's rude, you can't always blame the equipment!
I always consider the operator as being part of the measuring equipment. I agree that sometimes the fault is there. They are often the hardest to diagnose.
the problem is that one of the probes of my multimeter is faulty and it works from time to time. it works if i move the cable to a precise position.
at the beginning, i ruled out the multimeter because i always measure a "known" voltage to see if the instrument is working before i start measuring, and at that time it did pass the test. but i re-did the test and it didn't pass
now, i am using different multimeter and i got 3.3v and 5v.
thank you all ... i feel a bit stupid.
thank you all ... i feel a bit stupid.
Don't feel stupid, we have all been there done that at some time in our lives.
In the Air Force, working maintenace, we always had to document our findings on toubleshooting efforts. In a case like yours here, we would might have put down the following:
No problem found, suspect 'short between the headsets'. ;D
Lefty