Code (Script) Component error: The type or namespace could not be found

I'm trying to use a dll i downloaded (a file converter, in this use case from xlsx to xls) with my workflow project, but can't seem to make workflow designer reference to the dll/namespace/using. What are the problem-solving steps with the scripting component and namespaces?

Same problem has been presented (but not discussed) prviously here:

https://www-secure.symantec.com/connect/forums/adding-dllnamespace-code-scripting-component

Image may be NSFW.
Clik here to view.

Image may be NSFW.
Clik here to view.