I just purchased the KDFW scenery, is there a user manual available?
The manual can be accessed from the Addon Manager menu, on the FSDT Dallas page, pressing the Documentation button.
How do you activate the catering truck and service vehicles?
As with any scenery in FSX, you activate a push back tractor with the standard hotkey (which defaults to SHIFT+P), the fuel tractor with SHIFT+F and the baggage loaders by opening cargo doors of the airplane.
Catering vehicles at KDFW are static, but we are working on a separate product, called GSX, which will be dedicated to service vehicles, offering much better alternatives compared to the default ones, see this forum section:http://www.fsdreamteam.com/forum/index.php?board=45.0
GSX will work with all FSX airports, both default and 3rd party.