At this point it definitely seems to be an issue with your system and the landscape tool which is very GPU intensive. You should be able to tell if your system is equipped with the Nvidia GeForceGT 750m card by going to Apple>About this Mac, as shown in the first image above.
To set the Nvidia card as the default GPU for UE4, go to Unreal Editor>Preferences>Plugins>Graphic Switching: Select the Nvidia GPU, then set as default. (You will have to restart the editor for this to take effect.)
Let me know if this is A possible on your machine and B resolves the issue.