i have shoot. bas
So you have a script loaded that waits 2 seconds and takes a picture with automatic exposure settings.
Probably not what you want to do.
So the trick here is to exit CHDK ( press the <ALT> key again - the one you used to enter <ALT> mode ) and then press the shutter release. If you have setup things correctly in the CHDK menus, you should get your 256 second exposure.
Remember, when you press the shutter while <ALT> mode is active, CHDK runs the currently selected script. When you press the shutter button while <ALT> mode is not active, CDHK lets the camera take a picture but with the overrride settings you specified. Assuming you do not have overrides disabled that is.