/usr/bin/virtualbox, but it should also be in the Accessories section of the main menu (the menu section apparently can differ, see the above post)
if you have a 32 bit host OS, to be able to run a 64 bit guest operating system in VirtualBox your system needs to have hardware virtualization support present and enabled in the BIOS (see https://www.virtualbox.org/manual/ch03.html#intro-64bitguests - the documentation is a bit ambiguous about whether or not this also applies to a 64 bit host OS)