I was hoping to find answers on the forums or Google for my question, but I couldn't find any.
I was wondering, if I used a custom assembly for a plugin, do I have to do something specific with it when compiling my plugin?
I threw it in the managed folder, restarted the server, and tried adding the plugin. It seems like it can't reference it while compiling. 

You'd need to add a reference to the top of your plugin to include it.

// Reference: DllName

Thanks a ton! That did the trick