Go to to Khanacademy.org and login using your Google account. You may have to allow it access your profile if it prompts you. Once logged in, click on "Coaches" located on the menue bar.
Once the "Coaches" page opens, select the appropriate class code from below and enter it into the "Add a Coach" text box and click "Join the class". This will put you into my class where I can then assign and monitor work. You will need to be logged in and in the class to receive credit!
Make sure you choose the right period!