Not every event will have the payment plan option available. But if it does you can find it by adding the desired tickets to cart from the event page and clicking Buy Tickets. Once you get to  your check out screen you will find a blue button that says '+ Use payment plan - only pay $$$ today' under your order summary on the left hand side of your checkout screen.

Lastly, you will be prompted to accept the payment plan terms and conditions, agree to the schedule of monthly installments, and finally confirm your purchase.