diyaudio.com has some threads on this amp... at least one with volume pot questions/responses.
Frankly, I'd set the fixed gain level, using the pins you've pointed out,
as low as I could (both gain pins shorted to ground) if that provides
enough volume with the computer output maxed... and just use the computer
volume control.
That said, you -could- use a pot wired as a rheostat (
http://www.resistorguide.com/rheostat/)
to replace the input 10K and another in series (between the input jack and the DC blocking capacitor)
otherwise, when you turn the volume down you'll short the computer output to ground.
diyaudio is a much better place for such questions though... perhaps some better ideas.
Have fun.