It would certainly help to know whether or not you've tried plugging the guitar into another device, and whether that worked or not. You know...
verify that it's not the guitar.
First Act is not the best of brands. It's pretty much a toy, and famous for technical problems just like this.
Check the cord too. Try a different one. Buy a new cord and try it again. They're cheap enough, and you can never have too many cords.
Eliminate the guitar and cord as the problem first so we can be certain the problem is really with the Mac.
EDIT: The grayed out main volume in your Audio set up definitely doesn't sound right. Don't know if a messed up jack would do that, but I do know that using adapters the way you are tends to stress out and ruin the 1/8" jacks. I suggest using 90 degree 1/8" to 1/4" adapters instead of straight ones. The length of a straight adapter with a 1/4" inch plugged into it puts an incredible amount of stress on the jack, and usually will ruin them eventually. A 90 degree adapter still stresses it, but far less than a straight one will, plus you can tape it down so it doesn't move, eliminating most of the stress.