What is the file extension of the file you are editing? What language is it?
Try selecting some text and then go to MACRO -> RUN… and type in "Comment" (without the quotes) and click OK and see if that works.
Try going to TOOLS > CUSTOMIZE -> GENERAL (left side) -> FILENAME EXTENSIONS (right side), select the extension of the file you are editing, click ‘Edit’, check what it says for Language (top middle). Then click the "…" button to the right of that field, and we’ll have a look at the language setup for that language. The "Languages" dialog box should come up with your language selected. Click ‘Edit’ to get the Language Setup dialog box. Check the lower left where it says "End of line comments" and see if that is set right, and right above check "Block comments", and see if that is set right.