Looks as though you're trying to put the MongoDB connection details into where the Data Hub connection details go. The Toad for Cloud client first needs to connect to a Data Hub. If you don't have a Data Hub installed you'll need to do so. Download from http://toadforcloud.com/entry!default.jspa?categoryID=677&externalID=4372.
It is the Data Hub that then can connect to MongoDB.
You may find this post helpful, http://toadforcloud.com/thread.jspa?threadID=35235&tstart=25