MIDI control of a plugin

ACT (as plug-in) will not work in that case. You want "momentary" action (pressed - on, released - off). That is not supported
 
AZ Controller will work, and the only solution if:
* you want that more that in one track without reconfiguring anything (so, you focus track with this plug-in - you key works)
* you do not want that key "leaks" into soft synth
 
But if you just want do this for one track, in one project, Remote Control is simpler:
1) In the track, "Clips"/"Automation"/Instalooper/LoopX (where X is 1-4). Open automation lines.
2) On the right side of the automation controls, you will see a "box" (with "0%").
3) Right click this box, select "Remote Control..."
4) Press a key. Press "Learn". Select "Note On/Off" (not default!). Press OK.
5) Delete automation points (otherwise they will overwrite whatever you try to do with the parameter)
The latest Sonar should remember the setting in the project (some older versions was forgetting it)
 
For AZ Controller it is a bit more difficult (but once you understand it, you will be able to do similar configuration within a minute...)
1) follow "Quick start" (you will be unable to complete the task without that)
2) create new control and assign it to the button
3) in its logical configuration, set:
'Note:Any' - Strip Track <Current> volume
'Note:Any' - FX InstaLooper3 , FX Parameter (4 - Loop4)
(Note:On) - Value 1
(Note:Off) - Value 0
 
As you can guess, it will search current track for "InstaLooper" and toggle Loop4 when the key is pressed (and off when released). If you want this action on some Pad which send CC instead of Note, set Hardware control type to Button (in the options tab, for that control, do not forget to press "Save" after changing...).
 
In both cases, you can "Record automation" with the result.