Beyond Compare 4.2.4 Linux doesn't support automatically installing the context menu to launch comparisons from Files (Nautilus) in Ubuntu 18.04. Use the following command to manually install the context menu. Tested in Ubuntu 18.04 64-bit nightly preview build.

sudo cp /usr/lib/beyondcompare/ext/ /usr/lib/x86_64-linux-gnu/nautilus/extensions-3.0/
After installing the context menu extension, logout and login to make the context menu display.