Thought I would throw out what I learned today since finding Mac specific Borderland info is quite difficult.
I wanted to play the DLC Knoxx, but as I progressed I notice the sincere lack of outposts to make traveling between the areas easier. I mean, I read its a vehicle type DLC, but to take 30+ minutes to run back to where I quit (because I dont want to play for 4hrs at a time) having to kill enemies, or just run and pray...sucked.
So I found the Oasis and custom maps with which people have made outpost points inside the Oasis and then you can travel as you see fit. Makes The General Knoxx DLC, and the others without outposts im sure, much more tolerable.
I dont consider THIS cheating, just a way around having to just grind and run.
Anyway...on the PC its simple enough, any steps you read will be for Win/PC. To apply those same steps to Mac, you need to be able to WRITE to the Data Img which Borderlands creates when you open the app. Most mods require you to access some folders which are only inside disk images that are READ ONLY when mounted as you launch Borderlands. To fix this...
Goto the app folder inside your Applications.
Now open "BorderlandsData" with Disk Utility, and save it as a WRITE-able img somewhere on your computer. Backup your original disk image somewhere else on your computer, and put the newly saved one in its place.
Now, any steps you find online which deal with file/folder locations on the PC, more than likely they are on this disk image. Do what you need to with files/mods you find. When you save them to this new disk image, they will appear in Borderlands.
ps: this is for the retail disc version of the game, cant speak about this applying to anything else