credential fail, hp 5412ZL switch

I can use the RTBM with my 3com superstack switches just fine, but when I try to do the same thing with a HP 5412zl switch, I get credential failed.  I can walk the mib using ManageEngine's MIB browser just fine, so I know SNMP is working on the switch, and I have the right community string, but I want real time graphs.  Any idea how I can fix this?