Sorry about the confusion; that is only available for the newest POD HD devices as called out below. The PODxt's are 2 generations prior and will not have this capability, unfortunately:

Taken from the Mac OSX USB Driver Release Notes:

Driver 7.1.3 (Mac OSX) adds gain control for USB audio. You can now control the levels sent from your POD HD to your recording application using standard controls in the Mac operating system. These controls have been added for POD HD, POD HD300, POD HD400, POD HD500 and POD HD Pro.

The level control can be accessed in a variety of ways:

1) Go to Applications/Utilities, select Audio MIDI Setup, and select your POD HD device. Select the Input button to show sliders which provide up to +18dB of gain for channels 1 and 2. The advantages of controlling the level settings here is that you can address each channel individually which can be handy for POD HD, POD HD500, and POD HD Pro which are all capable of running two independent signal chains.

2) Go to System Preferences, select Sound, and select your POD HD device. Select the Input button to show a single input volume slider which provides up to +18dB of gain for channels 1 and 2. The advantage of controlling the level settings here is that you can address each channel with a single control which can be handy for all POD HD tones running in stereo.