What ArcGIS version are you using? (beta, pre-release, or final). This was an NIM for this issue, but it is reported being fixed in final 10.1 version.
[NIM068499 Tried running sample SOE manager property page code that worked in 10 in 10.1 and it cannot compile as there is no current ESRI.ArcGIS.ServerManager assembly. This is needed since it contains the IServiceObjectConfigurator interface that needs to be implemented.]
Version Fixed: 10.1