Hi Kevin,
To me it doesn't look like this is a problem with an incomplete install. Rather, I think it's related to the AD Connection Profile information you are using. Some changes were made to the workflow build that underlies ServiceDesk 7.5 regarding AD Connection Profiles, and this may be complicating the issue.
If you open the Credentials Manager, and edit your AD Connection Profile, what are you using for the Domain Controller and domain fields? The Domain Controller field is new to 7.5, and must resolve correctly for AD Sync to work. It was added to allow customers to specify which DC to connect to if there are multiple DCs in a certain environment.
When you've entered your Domain Controller, Domain, Username, and Password - Credentials Manager will automatically run a connection test. Does this pass, or do you see the same error here? If you get an error, try using a different method for addressing the Domain Controller or Domain (fqdn, IP address, etc). The specific error you got is a common .NET error, and I found several instances online where users resolved it by using a different method to address their domain (http://stackoverflow.com/questions/8086322/userprincipal-findbyidentity-insists-there-is-no-such-object-on-the-server for example)
Hopefully that helps you resolve the issue. If it doesn't, give us a little more detail on your Connection Profile and we'll see if we get diagnose it further.
Good luck!