What a neat little device! Where did you get it? Cost? Can a hobbyist buy just one or two? I don't want to order 50 from Panasonic!
The good news is: You should be able to hook it up okay to the Arduino. I know you said you have the digital type... I'll come back to that in a moment.
For other readers of the post, who don't want to go off to the datasheet:
This is just the "heart" of a PIR detector, but with some on-board smarts. Connect Vcc (4.5-5.5 suits either digital or analog) and ground. The only other connection is the PIR's output.
In the analog unit, this output comes from an op-amp, and goes directly to anything like the Arduino analog inputs. The voltage varies with how much movement the detector sees.
The output from the digital unit comes from a device I don't know the name for... scr? triac? (There's a note which suggest it may be a comparator.) Anyway... the "suggested circuit" diagram shows that it should not be asked to source or sink more than 100 uA... shouldn't be a problem if the device is connected to an Arduino digital pin, and that pin is set for input... input to the Arduino, output from the PIR. (Yes, I know... some of you understand that, but it is a real pain for new electronics hobbyists, remember?)
So, Lycrois, why isn't it working for you??
If you have a voltmeter, try monitoring the voltage on the output while everything is connected up. It should swing most of the way to zero or most of the way to 5v, depending on whether the PIR is seeing movement. (Put it under a ceramic coffee mug or something when testing for the "no movement" situation.) The voltmeter will TELL you want the Arduino is "seeing", and any bugs in your monitoring code can be worked out after you get the electronics right.
Be sure you've put...
... in your setup function... even though it should be INPUT unless you make it OUTPUT... but be sure you DIDN'T do that!
If the output, while connected to the Arduino, is always around 5v, try connecting a pull down resistor. (More on this in a moment.) If it is always zero volts, try a pull up resistor. (There's a clever way to avail yourself of a pull up resistor inside the Arduino, but that's a luxury for later!)
Pull up/ down resistor.....
Some devices, maybe the PIR is one of them, are good at making one of their outputs "high" or "low", but not good at making them the other state. Such devices need help.
That help comes in the form of a largish resistor... I'd try 10k, other readers may be able to give you more informed guidance on a suitable value... just don't make it too low! I wouldn't go below 1k, and maybe some kind reader will jump in and discuss "too low" further...
ANYWAY... connect the resistor between the data line (the output/input) and either 5v (for a "pull-up" resistor) or zero volts (for a "pull-down" resistor.
Like that, you should see the voltage on the data line go high or low (or is it the other way around?) when the PIR sees/ does not see movement.
Once the voltage is doing what it should, then you can start working on the software in the Arduino to "do things" with the information!!
Footnote: If you don't have a voltmeter, the following is so simple that it MAY not have problems to haunt you, mislead you....
Start with an Arduino set up with one LED, one switch. The switch connected to a digital input.
Write a little program to turn the LED on when the switch is closed, off when the switch is open.
Be sure that is working properly.
When it is, change JUST the connection of the switch. Connect the PIR where the switch was previously connected. Just the PIR (it being connected to Vcc and ground, too, of course) and see what you see. If LED always on or always off, try a resistor. You can connect it as a pull-up or a pull-down, and if it doesn't work one way, it might the other... but should not... in the instance we're discussing... do any harm if not too small and connected the "wrong" way at first.