Each of the two metal "arms" (thin, silver metal rods) that support the BBG have a small hole near the end. After you slide the BBQ on to those two arms you should be able to slip the pins through those holes to keep the BBQ from sliding off.
aaaah just got the trailer out of storage and looked at the bbq and bracket. i didnt realize that the bbq slid onto the arms then you used the pins to hold it in place. i was placing the bbq handles on the arm,s and could,nt figure out how to hold it in place with out it sliding all around . it even fell off the arm ,s once. thanks