[PyMOL] ImportError: No module named _cmd

[<and...@wr...>]

I've trying to make an external GUI for pymol and I'm new to python. I try to 
import the module cmd as follows:

from pymol import cmd

But I recieve the following error message:

Traceback (most recent call last):
  File "GUI.py", line 7, in ?
    from cmd import _cmd
  File "/home/panderso/software/pymol/modules/pymol/cmd.py", line 48, in ?
    import _cmd
ImportError: No module named _cmd

Any input would be greatly appreciated. Thanks in advance.

Paul