mod_cluster requires just a few steps on the httpd side to get working; however, just in case you cannot get ahead with it, you can find some clues by allowing a verbose output on the administration console:
When setting this directive, you can get some information about the single modules loaded into httpd:
Should you find any module that exhibits an OK in the display page, you have at least a clue to investigate.
One more possible cause of errors is advertising messages that could potentially hit a firewall. Remember that advertisement messages use UDP port 23364 and multicast address 184.108.40.206. ...