I was running into the same issue - and although many suggest that you can create a folder and point Blender to it (by going to
File > User Preferences... > File and selecting a location for scripts), I wanted to use the default location. I ended up finding this thread. It suggests that you go through the Applications folder and right-click Blender.app for
Show Applications Contents.
Using this method, you can navigate to
and place whatever addon folders you like there! Note: potentially obvious but 2.78 is my version and may be different for you.
The only other gotcha for me was that once I found my custom addon in
File > User Preferences... > Add-ons - Disabled and enabled it, I was forgetting to actually save this updated preference at the bottom of the preferences window. If you ask me, ticking a package to enable it should automatically be saved - but that's just an extra 2 cents :)
Hope that helps!
Edit: typo and clarification