I got a card to give to security from the stoma nurses here, explaining that I have a 'medical appliance'. I dont know if they have that in the USA though.
It is my understanding that security staff are trained about ostomies though. Believe it or not, there's ALOT of us out there!
Dont worry, dont stress!! It will be fine, just explain it to them when you get there. You will go thru the xray anyway, so they can see its not an issue. Just tell them before you go thru. Quiet like.