99% likely that you are trying to start the VM as the wrong user.
Type this:
[email protected]:/$ cd ~
[email protected]:/home/chris$ ls -la
total 48
drwxr-xr-x 4 chris     chris     4096 Sep 22 17:51 .
drwxr-xr-x 7 root      root      4096 Jun 29 18:13 ..
-rw------- 1 chris     chris     5659 Sep 22 17:54 .bash_history
-rw-r--r-- 1 chris     chris      220 Dec  8  2010 .bash_logout
-rw-r--r-- 1 chris     chris     3116 Dec  8  2010 .bashrc
-rw-r--r-- 1 chris     chris      675 Dec  8  2010 .profile
drwxr-xr-x 4 chris     chris     4096 Jul 25 15:33 .VirtualBox
If you don't see a directory called .VirtualBox then the VM you are trying to start is not owned by the current user.