The main vbox packages are under “virtualbox-dkms” or “virtualbox-source” and “virtualbox-guest-dkms” or “virtualbox-guest-source”. You’ll want dkms. The ext pack is in the repo as “virtualbox-ext-pack” but it’s not in the same section as the other packages in synaptic(kernel and modules(contrib))
have always worked. However id suggest to reboot your device once installation is complete and Vbox will be avaiable in Applications -> system tools -> Oracle VM Virtualbox