You can't?
Message Assignment is used for exactly what you might guess, assigning to a message. On the other hand if you put an Expression shape into your Orchestration then you can simply perform a .ToString() on your GUID variable and you should be good to go.
If you're trying to assign the GUID to a message and need it as a string, simply create an Orchestration variable before hand, assign the .ToString() to that, and then use the variable in the Message Assignment shape.
Tim Rayburn
Principal Consultant, Sogeti USA LLC
http://www.TimRayburn.net