So in a nutshell you seem to confusing the default credentials for you RockMongo UI with credentials to connect to MongoDB.
By default on installation, MongoDB has no Authentication turned on. You can configure authentication if you so desire, and even add that configuration to your Rockmongo Setup so that it can work on a privileged user.
All details are in the links provided.
For now just connect as follows:
Provided you have access from your or another machine.