This project (like angular/angular) doesn't do simple merges. It uses rebases and squashes to keep a simple, linear history.
Now that GitHub lets you squash as you merge, this page is less necessary. If you're accepting a change as-is, with only commit message changes (if any), then you can push the green button.
For those other times... These notes are based on Ward's setup, and Kathy has followed them many times. Many other possibilities exist (e.g. see Using Git with Angular repositories),