You don't get strongly typed properties for custom properties added by configuration. There is a code example at the bottom of the article User Profile Configuration that shows how to get the property value.
You should not be modifying mojoPortal code in ConfirmRegistration.aspx or anywhere else, see the article Avoid Forking the Code
If you want to do something with that custom property after registration you could implement a custom User Registered Event Handler that can plugin without forking the mojoPortal code.
Hope that helps,
Joe