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.
Solution
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) > Nation Defaults > Permission Sets > edit the permission set in question and tick "API" (and sign into apps):
The other permissions can stay the same.
For more info and a detailed tutorial, see: FieldEdge and NationBuilder Permissions.