You should be able to visit the memcached resource page, go to the Inventory tab, expand Configuration Properties, and click on the Edit button. Replace "localhost" with your IP address as the hostname, and click OK. Within a few minutes, you should begin to see metrics. If you do not, verify that your view range is set to a sane value (like 8 hours)...sometimes I have things cranked out to view 48 hours or further out, and I don't see metrics I expect with new services.
If memcached was auto-discovered by HQ, the IP address should already be the hostname. Not "localhost". If things look right already, you may be having a problem with the plug-in. If your issues persist, let me know what version of memcached you are running. I am currently testing with version 1.2.2.
