When logging in with the admin@system-domain user the Access --> Role Manager "vCenter Server:" dropdown in our environment is blank as well.
This looks to be normal and due to permissions assigned in vCenter itself...
Try this to get the vCenter Server field populated in the drop down
1. Go to SSO Users and Groups and select the ___Adminstrators___ group.
2. Edit the group then add your domain or local user accounts here that should have Administrative access to vCenter.
3. Log out and login with one of these accounts instead of admn@system-domain
The Access --> Role Manager "vCenter Server:" should now be populated with your vCenter Server name.