Because I used the same package but it works on Window 8. And I copy the project folder from Window 8 to window 7, it doesn't work with the same error.
These are my asset version:
-Leap motion SDK for window Leap_Motion_Installer_release_public_win_x86_2.2.7+30199_ah1876
-LeapMotionCoreAsset_2_3_1.unitypackage
-Window 7 64 bit
-Unity 5.1.1f1 or Unity 5.1.2
Also, I have this Unity alert like this before face to issue cannot found dll
And if I cancel, Unity continue with this alert: