Go to your game page.....to the right you will see your avatar and 8 little windows with your clubs and Ball showing......under that is a tab greyed out CHANGE EQUIPMENT.
Hover your mouse over it and it will go green (light up) left click it to open another window with all your equipment showing. Near the bottom right side is the CLOSE tab and above that is BALLS....left click on the BALLS.....and you will see all the balls in your equipment.
< arrows either side> to move through all the balls you have. Select the ball you need when it is in the middle of the three shown (if you have more than 3 that is) left click it and hold then slide the ball of choice up into the space for it top right.