MongoDB - admin user not authorized

I was also scratching my head around the same issue, and everything worked after I set the role to be root when adding the first admin user.

use admin
db.createUser(
  {
    user: 'admin',
    pwd: 'password',
    roles: [ { role: 'root', db: 'admin' } ]
  }
);
exit;

If you have already created the admin user, you can change the role like this:

use admin;
db.grantRolesToUser('admin', [{ role: 'root', db: 'admin' }])

For a complete authentication setting reference, see the steps I've compiled after hours of research over the internet.

It's a bit confusing - I believe you will need to grant yourself readWrite to query a database. A user with dbadmin or useradmin can admin the database (including granting yourself additional rights) but cannot perform queries or write data.

so grant yourself readWrite and you should be fine -

http://docs.mongodb.org/manual/reference/built-in-roles/#readWrite