[XeTeX] LaTeX Beamer

[Ulrich Groh]

How can I combine XeTeX with Latex beamer? I started with a document and

\documentclass{beamer} ..

Running the document with latex was ok. If I am using xelatex, then I  
got the error message

! Package pgfsys Error: Driver file ``pgfsys-xetex.def'' not found..

See the pgfsys package documentation for explanation.
Type  H <return>  for immediate help.
  ...

l.847 ...ver file ``\pgfsysdriver'' not found.}{}}

Using Google I got no understandable suggestion how to proceed. I am  
running MacTeX latest version under Mac OS 10.5 .

Thanks

Ulrich