You'll need to get the plug-in versions of Amplitube and Guitar Rig. Garageband can use AU (Audio Unit) plugins. Both Amp and GR come in the Audio unit variety.
Once you've installed the plugins, in GB create a new 'real instrument' track.
In the Inspector click on the Edit Tab.
Click on one of the empty Effects boxes and choose Guitar Rig or Amplitube from the list.