If the files sit in the shared folders (i.e. if you try to access it from within virtualbox, but they still sit on the regular filesystem) you might not be able to change the permission. I'm not familiar with it, but I remember from a long time ago (that was with virtualbox on windows) that you had to mount the shared filesystem in a very specific way and also add your linux user to a specific usergroup to be able to change the permissions and owners accordingly.
The simplest thing would (probably) be to copy the files into the linux file system and not try to get it right on the shared folders.