You can get the public key token by using following command. Keep in mind that the dll should be a “Strongly Typed” one.
c:\Program Files (x86)\Microsoft Visual Studio 10.0\VC>sn -T C:\sdk\Microsoft.Xr
m.Sdk.dll
Microsoft (R) .NET Framework Strong Name Utility Version 4.0.30319.1
Copyright (c) Microsoft Corporation. All rights reserved.
Public key token is 31bf3856ad364e35