I just installed vSphere 5.5 in my lab. It was a fresh installation. Not an upgrade. My vCenter server was already in my domain and I expected to be able to log in with my domain administrator account. Unfortunately it was not the case. To solve the issue I wanted to log in with the vSphere Web Client to validate my permissions and that my domain vclass.local was an identity source. In vSphere 5.1 the SSO administrator was called admin@system-domain this is no longer the case. You need to log in with administrator@vsphere.local and the password you defined under installation of the SSO server. When I logged in with this user I was able to configure my domain as an identity source and give access to my domain administrator to the vCenter Server.
Another thing I noticed was that the administrator@vsphere.local was administrator on the vcenter. In 5.1 admin@system-domain did not have any vCenter permissions set.