I've just plugged my mac back into my interface and Guitar Rig Kontrol after recording elsewhere, but when I go to click record in the Rig plug in track, a message comes up with 'No input source selected'.
I've gone back and checked all the audio and project settings and it all looks exactly the same as before, and the guitar's plugged in fine. And the input bar in audio settings, set as Rig Kontrol/Fast Track out displays the signal - it seems to be 'selected'. Is there anything else I'm missing??

2017: it’s in the Track controls (where the sends and reverb and stuff are, double-click on the track header) next to where it says “No Input” (or possibly input, depending on what you have set up). There are two intersecting circles. Click that and it becomes one circle. This works even AFTER recording, I just checked.

Garageband Preferences

• To change the input format, click the Format button to the left of the Input menu. Drag the Recording Level slider to change the input volume level for the sound source. If the Recording Level slider is dimmed, you can’t change the input volume in GarageBand. In this case, use the device’s own volume control.
• You need to select an input # (corresponding to the input # where your instrument is plugged in on your audio interface) on the channel strip, right below where it says 'I/O' (Input / Output).