As above, it sounds like you understand the concept, I will repeat just in case, two changes are required:
1. Edit the properties of the door and where USE KEY says 'key', rename to key3 (have not tested underscores).
2. Select a Key entity from the library and drop it in your level. Edit the key entity, and the NAME at the top rename it to key3.
Now run your levle and collect the key, then go to the door and hit RETURN to see if the door opens. Make sure the FPI script of the door MAIN AI is doorkey.fpi
"We are the knights who say...eki eki eki fatang loopzoing, zanziga....ni"