I noticed this seems to be a common problem for some mac users. I didn't go through the itch.io app and think maybe thats the problem. I just unzipped and ran. I think the root issue comes from Macs new "App Translocation" security feature. I posted about it here:
Sorry for the repeat, hope what i found and posted in the above helps.
Hello, i'm having trouble getting AssetForge to recognize custom items on mac as well. I downloaded the buildings pack from here:
The custom folder doesn't show up at all.
My folder structure:
AssetForge Version 1.2.1
MacOS High Sierra 10.12.1