@bnsH
Thanks for the details. Your link doesn't work anymore, but I'm able to reproduce the problem myself. My earlier experience with Unity 5 on OS X was pretty smooth, so until now I didn't realize how difficult this is on Windows.
I tried clicking around everywhere, but so far I've only found a strange hack to get the 64-bit editor working with LeapCSharp.dll on Windows. First step is to copy the 64-bit Leap.dll and LeapCSharp.dll into C:\Program Files\Unity 5.0.0b14\Editor.
Next, I had to set the CPU type for the x86_64 Leap.dll and LeapCSharp.dll to X86.
It's very strange to require the files to have the incorrect CPU type, but I've run through this same procedure several times and it never works when set to X86_64.
Second step is to delete only LeapCSharp.dll and Leap.dll from the Plugins\x86_64 folder.
Third step is to quit and reopen the Unity editor.
By this point I can play a scene in the editor without hitting a DllNotFoundException. I can also export the x86 target, although not the x86_64 target because it complains about DLL conflicts as obviously those DLLs have been deleted from the project.
This testing was done with FlowerStage.unity from Playground.
What's worse, I was never able to get the Oculus plugin to work at the same time--even after importing their 0.4.3.1 asset which claims Unity 5 compatibility--so this hack isn't yet sufficient for any of our VR projects.
We'll work with our contacts at Unity to get to the bottom of this. In the meantime, I'm hoping you and others here may be able to confirm this hack gets things running in the editor.