The rack mount kit doesn't come standard when you buy the wireless system.
You need to order the Shure UA-506 for single units or the UA-507 for double ULX units. Google for it and you'll find several stores (B&H Photo Video, Northern Sound, Sam Ash, etc) selling 'em, generally around $29. However, Full Compass--
http://www.fullcompass.com/product/263139.html -- sells 'em for $23.
Good luck on your search.