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Short answer: Mods can't work on OSX at all currently, and probably for the foreseeable future.

The errors you've posted to me suggested it is not working at all on your machine, and maybe Mac's in general. Basically, the mod engine is trying to build a backup folder, and failing.  That's the first step to applying a mod, it makes a backup copy of all the files, then loads the backup copy of files, and applies the mod.  If it can't make the backup, it'll never actually apply a mod.

I just downloaded the OSX build of the game, and looking in that zip, I don't see how the mod engine could work at all on OSX.  The mod engine expects to find a "files" folder that contains all the .gd files, but the osx package of the game doesn't have that, all the resources seem to be packaged up in a pck file. So mod engine, tries to find script files in the files folder, and it doesn't exist, so it errors a bunch trying to backup things.  then when trying to apply the mod, it can't find the backup folder so errors some more and basically does nothing.

I'm not sure if there's some way to have the game not use a pck file on OSX or not? If it needs to be built differently or could be manually achieved? The mod engine could also potentially be able to detect things are in a pck file and instead iterate on the files inside there and be made to work, however without a mac to actually test this on, it'd be a bit difficult to achieve.