Eerst moet je de Oracle VM VirtualBox Guest additions installeren.
Daarna moet je jezelf aan de vboxusers groep toewijzen
I had the same problem and fixed it by clicking in the VirtualBox group of my user. You can access it installing gnome-system-tools (it does not come with Ubuntu 12.04 Precise Pangolin), either via the Ubuntu Software Center, Synaptic or by typing in the terminal:
sudo apt-get install gnome-system-tools
Remember that to paste in the terminal you have to use CTRL+SHIFT+V, as opposed to CTRL+V
You will probably have to enter your password to allow the installation and add a Y (as in yes) to finish installing the packages.
Then you head to your Dash home and type users. You will see two applications. The good one is Users and Groups.
You then have to click on Advanced settings for your user and enter your password.
Now you will be shown a window with three tabs. Click on User Privileges. Find the line that says Use Virtualbox virtualization solution and then OK.