1) Click and hold the Material Editor button on the main toolbar - it should "flyout" with 2 options. Top one is the Compact, lower one is Slate.
2) Standard Menu > Rendering > Material Editor > Slate Material Editor, or Enchanced Menu > Create/Edit Materials > Slate Material Editor.
3) Both have a "Modes" Menu item which allows you to switch between them.
Max 2016 (SP1/EXT1)
Win7Pro x64 (SP1). i5-3570K @ 4.4GHz, 8Gb Ram, DX11.
nVidia GTX760 (2GB) (Driver 430.86).