I have this exact same problem. I don't see any green grids.
I'm using Allbest #37 for my A640 and trying to run MK3's Motion Detection scripts from Barney Fife's post
Highest Speed Motion-Detection (Lightning) Scripts.
I saved the text files per the
FAQ/Mac - CHDK Wiki guide just fine.
Then I moved the files over to my 8gb SD card scripts folder and put it back in the camera.
I start the camera and run CHDK in play back then go into the scripting parameters and load the .bas file.
The -current script- will change to mx3a.bas and everything else is at defaults.
When I go to shoot mode I have the camera in manual with flash turned off and focus set to infinity.
I turn ALT mode on and MX3A.BAS and <ALT> are on the bottom of the screen.
I push the shutter button full to start the scripts and I get a red error message top left that reads uBASIC:1 ukn stmt
This happens with every variation of the motion detection scripts I've ran. Any tips or suggestions. I have to be missing something.