Are you using your Browser or the one built in to Steam, as I think you'll find you need to use the mogstation link in the Steam app to pay that way.

Otherwise you can use your Browser as normal, go to the mogstation site and login, note that if you do this, you will limited to adding Crysta, using a gametime card or an accepted credited card or debt card to pay. You wont be able to use Steam payment directly, as I mention above to use that, go through Steam app and its browser.