How can I add a payment option using Square? All the options other than paypal are greyed out.

Please take note that you can only use 1 payment processor on each form. I checked your form and I can see that PayPal is already integrated into it.

You need to delete the PayPal field first so that you can add the Square payment processor. Here's a guide on How-to-Integrate-Form-with-Square.

Hope that helps. Thank you.