This is actually expected behaviour.
You are running a pre Update 1 Build of vCenter 5.1 which does not include a plugin for the Webclient for Update Manager. This was introduced in the Update 1 Build but is not completely functional yet, as it can not stage or remediate hosts.
More information can be found in the Update Manager Release Notes:
VMware vCenter Update Manager Release Notes
Update Manager Web Client is a plug-in to the vSphere Web Client. You can use Update Manager Web Client for viewing scan results and compliance states for vSphere inventory objects.