I also use mongov. Just do 'rhc port-forward'
Then run mongovue using the Mongodb credentials that OPENHIFT provided you.
Steps
rhc port-forward <app-name>
Wait a couple of seconds. You should see something like this.

Then run Mongovue. Add a connection and enter your Openshift Mongodb credentials as follows:

Save, select your connection and click connect. It should work.
You should provide you the mongodb credentials when you added the mongodb cartridge. You can also use ssh in your application and make "env" and see your password and mongodb username.
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