Winger, I've got the older style bunks, but on mine, there are two straps on each end of the mattress with snaps on them. The mattresses fold and the straps wrap once around the tube frame that holds the tent end up. Then snap into the bunk.
I'm not sure if this is the same kind of bunk as is on your's though.
DW, 3 Kids and our Goldens
2012 Shamrock 233S
2008 Toyota Sequoia 5.7L 4WD