Author : Doreen Kasson
URL : http:////docs.vmware.com/en/VMware-vSphere/5.5/com.vmware.vsphere.install.doc/GUID-5C6223B4-3E9E-4DD...
Topic Name : After You Install vCenter Server
Publication Name : vSphere Installation and Setup
Product/Version : VMware vSphere/5.5
vcenter active directory authentication setup
Check if the following article helps:
For vCenter Server 5.5, see the following for setting up a vCenter SSO identity source which can be a native Active Directory:
Add a vCenter Single Sign-On Identity Source
I'd also recommend you to upgrade to VCSA 6.5/6.7 since 5.5 it's not supported anymore.