Pretty sure Grey Havens wants monies, so fixing this issue might be pertinent to them.
1) Overlay works only on older Mac OS X, not the latest one.
2) People can't use steam funds if overlay doesn't work.
3) The issue causing this is an outdated libsteam_api.dylib
The fix is having a grey havens employee spend 10 seconds copy pasting a new dylib from the source SDK into the native folder.
Namely https://github.com/ValveSoftware/source-sdk-2013/blob/master/mp/src/lib/... being copy pasted into the directory fixes SK not being able to work with overlay.
Steam overlay needs to be fixed by Steam, SK devs can't fix that. I had to log on a Win computer to use it.