Right click in Ubuntu Virtual Box guest VM is not working
When ever  I'm setting up a guest OS, I find it helpful to do two things.


Open up the OS X system preferences and review every setting on every tab of the mouse and trackpad, paying particular attention to multi button and multitouch gestures and shortcuts.
If you use a trackpad or touch device, connected multibutton mouse over USB to make sure that those settings are passed along to recognized


At that point, you will want to be sure any guest OS additions are loaded in the guest VM. That will make the guest most likely to receive the best input and have drivers on the ubuntu os configured to take advantage of the multi touch / right click events. You can always check the virtual box support forums once you have a specific set of OS versions - you might uncover a bug in the code or they decided to program things as they work intentionally.