hi there,
got the same Problem and here's the solution (at least it worked for me):
what I had: esxi 5.5 with an Adaptec 6405. I created an individual image containing the adaptec drivers (AACRAID Driver v1.2.1-40301) and installed esxi with it (Link). At that time I had the same problem with arcconf you all had.
what I did: like written on Adaptec Website (Link) I installed vmware-esx-provider-arcconf.vib and vmware-esx-provider-arcsmis.vib (contained in "maxView Storage Manager for VMware 4.x and 5.x", at the download section for your adaptec controller), rebootet and at that time arcconf worked. Funny enough, the current arcconf Version (arcconf_v1_5_20942) doesn't seem to have the "SETVMCREDENTIAL" option, so I used an older version.
I don't think this worked some years ago: you can even use the "real" storage Manager (GUI) nowadays when installing some services on a guestvm (see installation instructions pdf of the storage manager download). Didn't try this, I'm fine with arcconf.
hth, Berndt