The first thing you'll need to do is make sure that your PODxt is running the latest software version. As of this writing, that would be version 2.01. The quickest way to make sure is to go to the Line 6 Software Download Page and grab Line 6 Monkey. Install the drivers (if you haven't already) and make sure that Monkey says that everything's current. Software versions below 1.02 do not support USB recording.
After the driver installation and driver setup, make sure that “line6 Podxt” is selected in Windows “sound and audio Devices” as the default audio device for "sound Playback and sound Recording”. Now, open your recording software, open the audio control setup of the software( In Live this can be done by clicking on the "EXT" button tehn Audio Tab) and select the Line6 Podxt as the main audio hardware device inputs and outputs. Now that the Podxt is communicating with the computer you must monitor all computer audio from the Podxt via headphones or speakers connected to the Podxt. The Podxt audio control panel is located in the Windows Control panel as Podxt.