MongoDB's standard connection string URI format is:
mongodb://[username:password@]host1[:port1][,host2[:port2],...[,hostN[:portN]]][/[database][?options]]
So in your case, your host is ds053708.mongolab.com
.
If you're still having problems you can check out MongoLab's doc on troubleshooting connection issues. One of the first things I usually double-check is whether you're using your MongoLab account credentials or your database credentials. They are two different things and in this case you want to use the latter.
Of course, we're always happy to help at support@mongolab.com.