Click on the login button on the user bar in MoodleDocs (upper right) or here. Use your moodle.org username and password. A separate login is required for MoodleDocs and for Moodle.org.
Need a username and password? First request a new account at Moodle.org here.
MoodleDocs is open to anybody to read and search. You must be a registered user and able to login to MoodleDocs to edit most pages.