hi there;
assuming you clicked "Equip bag" when you purchased balls; go to your profile page...
click current equipment tab; a window will open; click "Edit equipment"; then click on the BALL picture; you will see the ball you are currently using with an arrow < or >.......
click on arrow and any other ball you have avail in you bag will show; if your new ball is there, click and drag up to the ball picture up by clubs and release the ball on the picture. if you have done it correctly the lower pictures will show the ball with a "Equiped Tag" under the ball and that is the ball you are now using. hope i was clearer than mud, explaning while very tired :)
hope it answers your ????