You probably need to add the .dll to the project itself.
Afterwards, I would reload the project; I've had issues with custom libraries that have just been added - sometimes they don't register properly as a valid namespace until you reload the project.
reecardo's response didn't load in for me before I saved - to clarify, my response is pertaining specifically to the Code (Script) Component, and likely does not apply if you're using an integration project for your code.