So far we're unable to reproduce this issue. Is your Mac a "Retina"-type model, or a high resolution iMac? We're wondering if this is an issue with high DPI displays. I believe you can test this by dragging out a marquee (rectangular selection) in the node network and seeing if it's offset from the mouse position. If you can test that and let us know the result it would be very helpful.
Additionally, if that does not appear to be the problem, you can try resetting your Custom Bindings and/or deleting the preferences file. To reset bindings, go to Preferences - > Input Settings, and if you have customized the bindings at all, you'll have the Reset Custom Bindings option available. If that doesn't help, you can try renaming the Preferences file, which will reset preferences. On a Mac preferences should be in Your user folder -> Library -> Preferences ->
uk.co.planetside.Terragen_4.plist Just rename that file then restart TG and see if the issue persists.
- Oshyan