I think I will save a few people some time by sharing this tip that is not covered in the FAQ. Assume that you have a working munin setup and you setup the cloud monitoring from mms.mongodb.com (and you should). All seems to be fine but you are getting “Host Unreachable” – even though the connectivity between hosts and specifically, between the agent and mongod is flawless, then it might be a name resolution issue.
sudo vi /etc/hosts
enter a hosts entry as it appears in the mms dashboard (for example if staging-host1 with IP 192.168.2.20 is appearing on the mms dashboard as not reachable, add it to /etc/hosts)