Can't Enter Trap Door in Zork

The earliest text adventures weren't known for their large language base or interpretation skills. Many times if a specific door or object needs to be moved through, you need to supply a direction.

As in comments, "down" should work here.

Just for comparison, I appear to have Revision 88 in my library, and this specifically states as an error message, "You should supply a direction!" rather than what you get in your revision.

I know this thread is old, but make sure that you MOVE the rug. If you just look under the rug and don't move it, you won't be able to open the trap door.