I'm not sure whether I have ever used Mouse Click or not, but here are some alternatives that I have found by a quick search of the Extension Commands file I created (it should be linked to, or mentioned on the forum somewhere):
Delta v1.4 mouse commands
Delta Wait Double Mouse X | Wait for double-click of left mouse button
Delta Wait Fire | Wait for fire button (J0?) to be pressed
Delta Wait Left Mouse | Wait for left mouse button to be pressed
GUI V2.1 has multiple mouse commands for workbench based mouse work.
Craft v1.00
=Hw Mouse Key | Returns the status of the mouse key, even when AMOS is put 'to back' (eg. Amos To Back).
JD Extension v5.9
=Jd Moff Click | Returns a value depending on which mouse buttons are
| pressed (works even after Jd Multi Off has been used).
| 0=No mouse buttons
| 1=Left mouse button
| 2=Right mouse button
| 3=Both mouse buttons
Intuition v1.0
=Wb Mouse Key | Returns the state of the left mouse key.
| -1 Left mouse key unpressed
| 1 Left mouse key pressed
Anyway, the file is currently here:
http://www.geocities.com/aliensrcooluk/public/AMOS_ExtCommands.txt @ SamuraiCrow: The intuition extension has mouse commands that return the exact position of the workbench mouse. I'm not sure I can use the extension with my setup, but it must be possible to retrieve the coords of the WB mouse in AMOS

Regards,
Lonewolf10