We've got an easy solve for you: just download the Paw Points app. Entering the code through the app is a snap. Or go to freshstep.com/paw-points/ on your desktop and enter the code there.
If you still want to go to the Fresh Step site on your phone, make sure you're using the latest version of your browser (or maybe switch to a different browser). You might be using a browser that we don't support anymore. Remember, kitties like new shiny things. Now that's solved, time for a nap.