Blake Rudis has just posted a video tutorial on using this new feature.
Things to note:
1. This has replaced traditional Split Toning, but your split toning presets will still work.
2. This is not a replacement for your color grading tools in Photoshop. Photoshop has many more features built for color grading efficiency, masking, blend modes, blend if, Opacity, etc.
3. Try using the sliders. I find the color wheels to be difficult to navigate with accuracy.