If you are running Orion NPM on a Microsoft 2K3 server there is a known issue with communication between ESXi 5.5 and Windows 2003 due to ESXi 5.5 using 2048 bit security certs which Windows 2003 doesn't support.
Microsoft has a KB which addresses this functionality:
http://support.microsoft.com/kb/948963
I was getting "Test Failed. Cannot login with selected vCenter or ESX credential". Hope this helps.
Case #708443
-Chris