The Cassini system software occasionally requires ALT+Click to be used to modify values in a table (Limit table, or Test Exec Binning). For some installations of VirtualBox, the software may not respond to this command, even though ALT and left mouse clicks continue to work regularly.
Work around this limitation by holding the WIN or OpenApple Key while holding ALT key and pressing the left mouse button. (WIN + ALT + Click)
Another work around is to disable mouse integration (HOST+I) and switch to fullscreen (HOST+F). HOST key is normally mapped to the right CTRL key.
(Source: https://superuser.com/questions/710091/how-to-use-the-alt-key-in-a-virtual-box-guest-session )