When I'm logged in as Admin, and click on "DELETE" in someone's message, MiniBB says I don't have permissions to do so or something of that sort..

I think you're trying to delete the topic message - it is the first message in thread.
You can't delete this with ordinary Delete message thing. You have to delete the WHOLE topic. You can find Delete topic link at the bottom of the page neaer the "Choose forom" button.

RTFM ;)

4 Oct 2006 08:19 - Attached on mergeing:
question (deleting topics)

I (obviously) am admin of my bbs. I recently tried to delete one of the topics that was create on my board, but I end up getting a message saying, "You are not allowed to perform this action." How can I grant myself access to perform this action?

Jasonr
Probably you are trying to delete the first message of the topic. You can not do that - you need to delete the whole topic (links below).

Thx again for the help!