It's been years since I've tried to download and use mods, and I think I'm overcomplicating the process (like I usually do). I got one mod downloaded, applied the plugin under Data Files before entering the game, but not only will the mod not work, it causes errors to pop up in the game. So what is this noobie missing (if not everything)?
I appreciate any help, and my apologies for such a question.

If it's only ONE mod, and it's causing an error right at startup, you may be lacking MWSE or a required ESM file.

What OS are you using?
Vista and Win7 need some extra (but simple) steps to make mods work.
Just http://www.gamesas.com/index.php?/topic/781497-vistawindows-7-problems/page__view__findpost__p__11352753.

If it's the Steam version of Morrowind, you need to redate the BSAs for some mods to work.

The pinned http://www.gamesas.com/index.php?/topic/106063-morrowind-mods-faq/page__p__1772866#entry1772866 also has the basic info.