I've bought today the Senheiser PC 3 Chat headset and couldn't make the mic work. I'm a mac user and have a m-audio Fasttrack Pro. I both tried to plug to the computer and to the sound card but I can't get any sound. Even the signal light on the Fasttrack doesn't light on. Why doesn't the mic work?
Well, it won't work (direct) into most modern Macs - there hasn't been a Mic preamp in a Mac since the days of old. The "audio in" jack, when present, is a line level input. Some simply have no input jack at all (you're expected to buy a USB interface if you want input abilities.)
If you plugged it into the FastTrack Pro, you would need an appropriate adapter, since it does not appear to have a 3.5mm microphone input. However, that also seems less than likely to work, unless the adapter also supplies the Mic with power to suit it. Given that the headset mic is (presumably, Sennhauser not being overly proud or detailed in the "technical specs" they publish) likely a plug-in power standard "computer" microphone it needs 3-10V (5V nominal) power supplied in the "ring" of the jack, which is not going to happen with a standard 1/8" to 1/4 inch adapter, and the FastTrack pro appears to be set up to provide 48V "professional" microphone power, not low voltage "plug-in power". So, you are going to need something that will fit between your Mic and your computer that's compatible with it...
If you happen to have such a thing, a portable minidisc recorder or solid state recorder or video camera might serve as a source of plug-in power for the mic and some conversion to line level for the Mac audio input. Or you could build (or buy) a preamp that's compatible with this type of Mic.
Here is some more reading on various types of microphone power - the computer type and the small audio recorder (stereo) type are not exactly the same, though they often manage to work well enough (at least at mono) despite that.