If your scripts fall into the Assembly-CSharp-firstpass-vs project, it means that somewhere up in the folder hierarchy you have a folder named "Plugins", "Standard Assets" or "Pro Standard Assets". This forces you to compile scripts in the first of 4 possible compilation steps. Therefore, Unity will put your code files in the Assembly-CSharp-firstpass-vs project.
To access 2DToolkit from your code, I would recommend that you move all your script files so that they are not placed under any of these folders. This will lead to their placement in the Assembly-CSharp-vs project.
If you use C #, it should work now. But if you wrote your own code in UnityScript (JavaScript) or Boo, you will need to place the 2DToolkit in a folder called Plugins (either "Standard Assets" or "Pro Standard Assets" if you prefer this), so it compiles before your code .
Take a look at the Script compilation page in the Unity manual for more details on how you can control the script compilation order.
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