Open the game client (play now button) and then left click menu. Left click my equipment, Then near the bottom where it say "DRIVERS", left click that tab. It will open and show all drivers in your equipment inventory. Use the green > to select one. Left click and hold it and drag it up to one of the 5 boxes under the "OPTIONAL CLUBS" heading. That will out the driver in your bag.
If you had a rental club, when the rental expires, it is removed automatically. You have to do the above to reload your owned clubs.
Good Luck