After adding of the MongoDB cartridge to your application you can not build a connection from your local client (MongoVue or Unit tests from your Application) because usings ports are closed.
Follow steps will help you:
1. Activate a port forwarding to your application:
rhc port-forward -a <your_application_name>
you will get a message like this:
Service Local OpenShift
------- --------------- ---- --------------------------------------------------------
haproxy 127.0.0.1:8080 => 127.3.6.2:8080
...
mongodb 127.0.0.1:55131 => <your_app_domain>.rhcloud.com:55131
2. keep the executing of the port forwarding and use local IP and Port (127.0.0.1:55131) of MongoDB for building your connection