Your MX420 appears to be one of the models supported in Mt. Lion. More information is available in this KnowledgeBase Article.
If everything else in your network is working ok, double-check the setup in your printer. There is quite a bit to set in the MX420.
And if you are using MAC address filtering in your wireless router, make sure the MAC address of your printer is included in the router's access control list.
There is also a network troubleshooting guide on Canon's support page for the MX420