DIAGNOSIS: This is a common frustration. The metal insert inside the pre-drilled hole isn't gripping the wood, so it just rotates instead of letting the bolt tighten down.
FIX: You need to apply downward pressure on the insert itself. Here's the trick: 1) Take the provided bolt and screw it into the spinning insert by hand until it's finger-tight. 2) Now, while pushing down firmly on the bolt head (this puts pressure on the insert), use your Allen wrench to continue tightening. The downward force will often bite the insert into the wood enough for the threads to catch and pull it tight.
IF THAT DIDN'T WORK: If it's really stripped, you can carefully apply a small drop of super glue or wood glue around the edge of the insert where it meets the wood, then immediately tighten the bolt and hold it until the glue sets. Be very careful not to get glue in the threads.
HEADS UP: Don't overtighten! These are machine bolts into metal inserts. Snug is sufficient. Over-tightening can strip the threads in the insert or crack the wood laminate around it.