The way Shopify's checkout cart is set up you will only see the option at the END of the checkout process.
Below are instructions on how get up 12 month payment plan on any product:
1) Go to item you want and click the buy now button. If the item is already in your cart just go to the shopping cart so you don’t end up with 2 in your cart.(unless you want to buy 2)
2) Once at the first checkout page DO NOT click any payment icons at the top of the page.
3) Go straight to the "enter email". Enter your email and all info below that. Once done entering all information click “Continue to Shipping”
4) Select the shipping you desire and the click “Continue to Payment”.
5) Look all the way at the bottom and select “Splitit: Monthly credit card payments -no fees” You will then be redirected to splitit to complete your order and be able to do up to 12 months of installments interest free.
Below is what it will look like