I can't log in or access my forms

I logged in here: http://prepopulate.jotform.io/

But then it is just stuck on "Loading forms" 

Hi,

Since your account is a sub-user account, you may need to upgrade your account to Free/Starter account before the apps will work.  Try going to this page:

https://www.jotform.com/myaccount/billing

Then click on the Starter/Free subscription and change your account to that.  Once you have made the change, you should be able to use the apps properly.