There may be a bug at the root of this problem - perhaps in Synaptic, but I can't see a forum for that. (I'm really still a Newbie).
After enquiring, I selected the new Virtualbox/debian feisty to download for Cassandra. This happened very quickly, but the package is only 2.3 kB, while the older Edgy version which I downloaded last week is 11.8 MB - strange. Before use I transferred it from desktop to downloads, as I routinely do.
R-click shows Open with MintInstall, so I did that - a slow process - progress box reported various difficulties on the way, but ultimately reported Finished, and later in another box I saw Installed. (Several tries at this process).
To confirm, and hoping to use it, I looked at All Programs in the balloon from Cassandra - not there!
Next I opened Install Software (Synaptic Package Manager) which did show VirtualBox - with a red mark - Marked for deletion. The reason for this post is that I can't remove that! The manual doesn't specifically mention Unmarking, but here it doesn't work.
I know a problem was reported with Synaptic about 2 weeks ago, and I ran into that while trying to use it with Bianca.
Hope this helps -
Yours sincerely, -