Can't Record Audio

I'm using GarageBand 2.01 with a PreSonus FirePod and am unable to hear my microphone to record vocals alongside an existing iTunes instrumental track of which I've successfully imported into GB.

I have a Shure Beta58 connected to FirePod Mic Input #8. The FirePod's PreAmp Gain Input #8, Main, Phones, & Mixer are all at 12 o'clock (50%).

My GB Track Info window is set to Vocals, No Effects, Channel 1 (mono), Monitor On.

However, the Track Info Volume slidebar is greyed-out. If this is the problem, how do I activate the Volume slidebar so that I can monitor the microphone and begin recording?

Any input would be greatly appreciated..............