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Open the terminal app and type that command in. 

This assumes the app is installed in the main Applications folder, you might have to customize it based on where the app is installed on your system.  If you're unsure of the proper path, right-click on the app and when the menu opens, hold down the option key and you should see a menu item appear named 'Copy "IslandPlannerMac.app" as Pathname". Paste that into the terminal window and append '/Contents/MacOS/*' to the path.

See my comment above; for Catalina, the only thing that worked for me was to chmod the executable.

Are you running Catalina?

If so, try running this command (update the app's location on your system accordingly):

chmod +x /Applications/IslandPlannerMac.app/Contents/MacOS/*

I was having issues with Modo obj export earlier. Asset Forge doesn't seem to like certain flags, like 'group'. Modo seems to want to always add a default group, so I opened the obj in a text editor and removed all group references and it worked fine.

.obj file format