Some of the given examples on the userAdd manual page are outdated and don't work on 2.5.3. To reproduce one example:

db.addUser( { user: "<user>", userSource: "<database>", roles: [<roles>] } )

The userSource field is not a valid field anymore in addUser. I believe that you now must switch to the database where the user's credentials should be delegated, then add the user from there (and this only works for $external, too).

use $external

db.addUser( { user: "<user>", roles: [<roles>] } )