MACINTOSH:
You can select the input source from the standard "Sound" Control panel or "Monitor and Sound" control panel.
You can also select it from the "Settings Bar".
WINDOWS 95/98/ME/XP:
The sound input selection is done in the "Recording Volume Control" window.
Find the "Volume Control" application (under Start->Programs->Accessories->Entertainment). Select the Options->Properties menu, and then select the "Recording" button and hit OK. You now have the "Recording Volume Control" window.
In this window, select the input source that you wish the asFFT to hear (CD for CD audio, line for the line input, etc..). Note that the "All" or "What U Hear" source corresponds to the sound produced by the computer itself.
Also check that the input volume is close to the top.