The Flair was designed to be used with or without the bolt. You should bolt the main piece to the base only if you do not expect to travel with the Flair often.
If you leave the Flair permanently assembled and you want to bolt the base to the main piece, first you need to make sure the bottom tab of the main piece is all the way in the base. You can tap the top of the main piece with something soft (rubber, plastic, wood) to ensure it sticks to the base. Then flip it upside down and insert the bolt.
It screws in a bit tilted in the beginning. Again, don’t bother using the bolt if you plan to take the Flair apart frequently. Please see the image below for the appropriate angle for bolt insertion.