Took me a minute to find it too. There should be a yellow note at the top of your roll page. You can also go here: https://freebitco.in/static/html/wof/wof-premium.html. Not the most intuitive, I know.

On a similar note, I bought the WoF spins by accident because I didn't even notice the RP option was gone and went by memory. Stupid on my part yes, but a popup for changes to the site or rules would be welcome.