well, I do the same thing, what I did is went to radioshack, unfortunately they didn't have any 1/4" mono to 1/8" stereo, so, I got a 1/4" mono to 1/4" stereo, and a 1/4" stereo to a 1/8" stereo, so, I go through 2 adaptors... total cost for both was like $3.75 USD so it's not very expensive.

and I think you've got the right hole. mine is next to the mic input... it goes like this on mine:

Right: line in, Middle: microphone, Left: line out

I have a dell pc, if that affects it any (I know only a little about computers)