Linux Mint Forums

can you see virtualbox using synaptic?
can you post your /etc/apt/sources.list
you could try using the software portal (mintInstall) to install it.

try to purge virtual box first then install again.

sudo dpkg -P virtualbox

install using the software portal.

synaptic sees all packages that are installed even if the repository they are from is not in your /etc/apt/sources.list.
mintinstall will add and remove a repository from sources.list to be able to install a package.