How do you get the Catering truck in FSX?

There isnt one. You need to get a plane addon with AGS (Automated Grounding Service). You can get the baggage truck and ramp by opening the cargo doors (shift + E + 2) if you are in a jetway or a parking spot with ground trucks.

well i don't think that's true because if you go into computer,program files Microsoft games, fsx, sim objects, ground viehicles, and scroll down a little into you find what it says CATERING or CATERING TRUCK it either says euro or catering truck grey and its in the game but youv gotta do somethin do make them work im still workin on it even. so that's just a little tip.