Ok, I will try to make it clear,
if you bought the board from Gianluca, I am pretty sure that he burnt the bootloader in the board.
The explanations about how to burn the bootloader are for those that just got a blank PCB, or those that want to make their own PCB, or even those that mount the standalone version on a prototyping board.
Then, assuming that you have the bootloader already installed in your USB board, let's go step by step trying to find your problem:
1) when you plug your board to the computer on the USB port, does the LED marked as PWR light up?
If yes go to the next question, if not, can you check out if the jumper SV1 is connected between pin2 and pin3?
2) have you installed the IDE software in your MAC?
If yes, continue to the next question, if not, download and install:http://download.berlios.de/arduino/arduino-0002.dmg
3) have you installed the drivers for the USB device in your MAC?
If yes, continue to the next question, if not, download and install:http://www.ftdichip.com/Drivers/FT232-FT245/VCP/MacOSX/FTDIUSBSerialDriver_v2_0_4.dmg
4) Now we are going to check out if the bootloader is installed in the board for real. To do so take an LED and connect it between the Digital Pin 13 and GND, do it as in the picture below.
Now reset your board pressing the pushbutton S1 onboard (it is just by the PWR LED). When doing so the LED you just mounted has to flash once.
Does it flash?
If it flashes once: Congratulations, your bootloader is installed. If it doesn't flash, try to flip the pins of the LED and reset again.
You have to make sure that the long pin of the LED (thus possitive) is plugged to pin13 in the board, and the short to GND.
Does it flash now?
If it flashes, now we are sure the thing is working and you can jump to the first example. If not, you chip is not having the bootloader.
- - - - -
After you have intalled all the software in your computer, including the drivers, and after you have made sure that you have the bootloader, you are ready to start with the examples.
Now, I can ask you, do you have the bootloader in your chip?