for a solution. Follow the steps in amagine's post.
You have to change system preferences for function keys. You can change it back later.
Perhaps you can also turn on special modifier keys for function keys. If your keyboard lacks a FN key try Shift + Apple.
It's always a good idea to search the forums using google.
A search using the keywords site:forums.virtualbox.org F8 Mac
produced the solution on second place.