Unity/C# Added assembelies, MonoDevelop builds, Unity won't run

Hello,

I am stuck, and angered, please help!

I needed to add a .NET assembly to my game. So in MonoDevelop, I went "Project" > "Edit References".

Under the "all" tab, I selected the appropriate assembly, and hit ok. Then in my code, right clicking and selecting "resolve" became available!

So far, so good!

So I finished the code, built it, and it succeeded with 0 errors!

Then when I went back Unity, to try and run the game, it said that all compiler errors needed to be resolved before it could run. (And the error was indicating something with the assembly)

So I build it again, 0 errors, wash, rinse, repeat. GRRRRR!

I'm assuming Unity somehow doesn't know about the assembly I added via MonoDevelop.
What did I do wrong, or how do you fix this. (I KNOW it's simple, but I can't figure it out!) Did I mention, GRRR!?

Thanks, Benji