Easiest way to do it is to click their bell amount under their username and enter the amount you want to send and then just hit "submit". However with the fair going on, you'll see instead of "amount" the window says "redeem" - to fix it change the drop-box from 'bells' to 'tickets' and then back to 'bells' and it should work.
There's a guide >here< that might explain it a bit clearer if you're still confused. x)