Before using Virtualbox on 64 bit hardware, you may need to enable VT-x in your
BIOS if you have an Intel, or AMD-v if you have an AMD processor.  These may 
be disabled by default esp in older machines. 

A relevant online post about this:

https://forums.virtualbox.org/viewtopic.php?f=1&t=62339

Then import Virtualbox:

sce-import virtualbox-qt-5.1.30-stretch

This will import Virtualbox and the needed kernel modules. 

Then load the extension and it should be ready to go.