Sounds like you need to implement a custom Tool: http://www.sketchup.com/intl/en/developer/docs/ourdoc/tool

You can make the tool select entities the user clicks on by using the PickHelper class.

Both these classes has examples in the documentation that will get you started.

Your error message pointed to the last line in your code:
classify (u, apoint, face1)

I suspect it was the space between the method name and parentheses. Especially when you encounter syntax error it's good to go over your code and make sure the code style is consistent. Checking that there are no spaces between parentheses in method calls and making sure you have parentheses around all arguments etc.