I can't seem to get the mod tools to load within unity, I'm having just about the same issue as this forum post:

https://www.simpleplanes.com/Forums/View/1549837/Error-in-start-creating-Mod

I've tried multiple Unity versions those being:
2022.3.6f1
5.0.1f1
and two versions of Unity 2018 I can't recall

I can't find "MacOS Support", but Mac Build Support (Mono) is available. I don't know if modding is possible on a Windows computer, but I would assume so.

I'm not sure which one is current for the modding toolkit. I'm getting the impression the devs are more or less done with this game for now. Seeing as a lot of the content for modding is obsolete by a few years and the modding tools folder doesn't have any text files to help clarify much.

I should also mention I am trying to either make a map or modify the base game's "Build and Fly" or sandbox map. I'm not really sure where to start with map creation seeing as there's multiple things to consider such as spawns, materials and terrain painting to list a few. Any help on how to do any of this and how to mod the game in general would be appreciated.