You are not starting virtualbox from the command line with enough parameters, so it doesn't know what you want it to do. Your code:will just start the graphical manager. If you want it to start your actual virtual machine, then you must also tell it what virtual machine you want it to run. If your VM is called "ExAmPlE", then, as deadflowr states:
...remember, case matters. The name must be precisely that of your VM.
virtualbox --startvm ExAmPlE
For some reason, when used in a desktop file, at least in my install, the switch prefix (--) must be left out for VBox to recognize the command, i.e.
Exec = virtualbox startvm ExAmPlE
Don't know why. Would love to have some guru clue me in.
Exec = virtualbox --startvm ExAmPlE
Setting up a desktop launcher involves some arcane twists. When you are ready, we can give you step-by-step instructions to get you set up. Just get it launching from the terminal first.