Huh. We just tried it again, adding the reference for Boo.Lang to all 3 Assembly Reference spots in the Mono solution, and it did not remove the error. We are using Unity 5.4.3f1, on both OSX and Windows. As a side note, we tried it on 2 separate Macs. edit for clarity: On Windows I don't see this error, and am using Visual Studio. On the 2 Macs, they are using Mono, and both see this error. To add the references, we opened the solution tab in Mono, and for each Assembly-Reference (Assembly-CSharp, Assembly-CSharp-Editor, Assembly-CSharp-Firstpass), we clicked Edit References and checked "Boo.Lang" in the list. We are using the UniFileBrowser.dll. I also made a branch for us to try on the Macs, which uses the JS file instead, but that didn't appear to make a difference. Does this match the steps you took to resolve the issue on your machine? We also tried checking the x86, x86_64 boxes under Windows and Mac for Platform Settings in Unity itself, although Any Platform was already checked.