A new build of MMEX v1.3.3 for mac has been posted. If you have been having issues with right-click menu items being greyed out, please download this updated build to resolve those issues. The updated build is named mmex_1.3.3_macos10.9-wx3.0.2build.dmg and is available on moneymanagerex.org, GitHub and SourceForge. It works on macOS 10.9 and above.
Thanks to everyone who reported the bug and for your patience while we got it resolved.
If you want details about what happened under the hood - MMEX is built with wxWidgets as the foundation. Version 3.1 of wxWidgets unfortunately has a bug which caused right-click menu items to be greyed out in MMEX. The new build of MMEX has downgraded wxWidgets back to 3.0.2 to address this issue. All MMEX code is the same.