Cannot install virtualbox extension pack

it's a package bug, fix to be released shortly: https://bugs.launchpad.net/ubuntu/+source/virtualbox-ext-pack/+bug/1605799

just wait a few days, update and try again.

I had the same problem
I went to:
http://download.virtualbox.org/virtualbox/5.0.24/Oracle_VM_VirtualBox_Extension_Pack-5.0.24.vbox-extpack
I selected open with VirtualBox
When the download finished VB ask to install, I accepted the license and it installed correctly.
Copy the link and paste in your browser and you'll get this:

When finish download VirtualBox opens with a dialog to install it and then you click install and you get the license agreement:
Accept it and it will finish the installation

You must use the version of extension pack from the official site. Just check your vbox version before downloading.

But wrong package blocking dpkg, so that was a problem.

To solve that just remove this package using

sudo dpkg --remove --force-remove-reinstreq virtualbox-ext-pack

and then update

sudo apt-get update