It is an Inspiron 537
The modem should be near the bottom of the case.
There are four rectangular panels, and one of them should have TWO RJ-11 jacks in it. You plug your phone line into one of them, and optionally plug your phone into the other (so you can use the phone when not FAXing).
Look closely at the two plugs (you'll probably need a flashlight), and one of them should be labeled "line", and the other "phone". If so, plug the phone line into "line" and the phone into "phone".
Others have suggested you need a FAX program. I don't use Vista, so I don't know if that's necessary. All I have ever done is print to a FAX device from the program I'm using (i.e. Microsoft Word).
If you don't already see the FAX device in your list of printers, make sure that it's enabled in the device manager. If you need instructions on how to do so, follow up and I'll dig them up.