This usually means that you have not registered the DLL, or the dll is missing from the system.
Make sure the dll is in the proper location and then register the dll with regsvr32 like this in a command window, make sure you run as administrator!
Also, if you are running this as a classic app on IIS7, make sure you enable the app pool to run in 32 bit mode... Click here to see how
regsvr32 D:\Domains\uesdev\components\UESGasManagement.dll