This error usually means that the assembly was not found. Try verifying that the file exists in the directory where your application is running.
Try these steps:
--> Copy DLL to some other location as on Desktop.
--> Delete reference from project and also delete the DLL from BIN.
--> Copy the DLL to project root.
--> Add reference and choose the DLL from root.
--> Build and try running.
--Amit