When you add an outside source then you need to add their key. This is for
Synaptic. https://help.ubuntu.com/community/Repositories/Ubuntu Look at the
section Authentication Tab. You can also download and add it from the
commandline. See: https://www.virtualbox.org/wiki/Linux_Downloads Look for
the Public Key section.See the line that begins with wget. Copy and paste
the line into a terminal.

This will fix your error, but not necessarily your problem.