we could download your config.ini file [via these instructions|http://nam-en.apc.com/cgi-bin/nam_en.cfg/php/enduser/std_adp.php?p_faqid=8789], to save and look at your settings, and then try resetting the PDU to defaults (except/excluding TCP/IP) to see that it works, then reupload the file afterwards so you dont have to reconfigure everything.
before you reset it, maybe you could take a screenshot or post the SNMP portion of your config.ini file as well as the settings in your NMS just to get a second set of eyes looking at it to to make sure there are no typos, etc