If you are receiving an error message along the lines of "You do not have permission to use this app"...
...it is because the account you are logging in with does not have the correct permissions.
Ask an Admin of your nation to change the setting of your permission to enable "Sign into apps".
They can do this by going to Settings (in the main menu) > Defaults > Permission Sets > edit the permission set in question and enable API – sign into apps:
The other permissions can stay the same.
For more info and a detailed tutorial, see: FieldEdge and NationBuilder Permissions.