The workaround to access the asset store when using mac OS Sierra is to drop the tab to a single window. Than you can see the asset store. Unity or Sierra don’t support to display the asset store in the tab (embedded window).
@MASTware Your workaround works, but I don’t think it is just a Sierra problem. If I open my copy of Unity3D version 4.6.2 (which is a Personal/Free version) in Sierra, Asset Store opens just fine as an embedded window.
Yeah im having this problem only I can see it for 1 sec then it all goes grey until i scroll or resize the window. then goes grey 1sec later again. But if another program like chrome had focus I can hover over asset store in editor and scroll and read it fine no issues but soon as I give unity focus again it goes grey.