I just went out and got a 1/4" to 1/8" adapter so I could record on my computer using Audacity, and I'm running into problems.

At first I plugged it into the headphone slot and the amp outputted my PC sounds. Fixed that one pretty quickly.

But now, I'm definitely in the microphone slot and I can't record at all. The mic slot definitely works because I can use it with my normal mics. My microphone and Line in volume are at full, so that can't be it. Its definitely also not the cables because these all work great through my amp..

My amp is a Peavey 5150 and I'm trying to use the preamp out through the head into the 1/4" side of the adapter.

Should I be going through the cab's parallel mono 1/4" or am I missing something here?
mostly your computer doesnt have the driver to run it as a input if you dont have a midi board i dont think its gonna work
That made no sense whatsoever. I mean I run a pretty decent computer, its not old at all or anything. Win XP SP2 with a fair audio card.

I don't get how it wouldn't work when it works with normal microphones from the same slot.
First of all, when using the Line Out make sure you've got that head hooked up to a speaker load. You do know it's not going to sound that great, right? You're getting sound from the preamp, so it's going to sound thin and harsh. You're best bet would be to mic the cab or use a speaker simulator with the Line Out.

As for no sound when plugging into the mic in... what mic are you using? There's a good chance the mic needs to be plugged into a mixer or some device with phantom power.