I've noticed quite a few "maybe someone on Mac can help" comments to, well, a lot of things, so I'd like to shed some light on how to work with mappacks (and touch on modding a bit) for those of us using OSX.
I'm using OSX 10.7.2 (Lion,) but these should work on most builds.
First you need to identify where your LOVE library is, if you even have it to begin with. I don't have the link handy at the moment, but that'll be in a future edit. When you run Mari0 for the first time, it spawns a folder in Users/<You>/Library/Application Support. Your mappacks folder is in here. If you don't have the LOVE build, this is the place to put it after you move it out of Downloads.
There will likely only be the mappacks folder and options.txt files in there, maybe some extras if you downloaded a mod.
For most mods, all you'll usually have to do is run the .love file and that's it. Editing the mod requires first changing the extension from .love to .zip and examining each individual .lua file by hand, then zipping it back up, changing the extension and running it to see if it'll work.
An example would be replacing the tileset with a different one you have on hand.
Here, I've taken the tileset from the Super Justin Bros. mod and replaced the Extra Entitles tileset with it.
Finally, once you've made your edits, it's important to zip the files up from within the folder, because if you zip the folder itself, you'll get an error saying main.lua isn't in the root directory.
Hope this helps a bit.