If you have more than one degree, go to "My Profile" and "Edit Profile". Scroll down and you'll find the "Education" section.
As you can see, under the current education, there is an "Add Education" button. Click on it and fill in your master. If the master or any education has been completed outside of the US, please see:
If you are currently attending your bachelor, master or PhD, in the "Degree" part, you'll see: Associate, Bachelors, Masters, Doctorate, and Enrolled. Choose Enrolled and the year that you enrolled and add in your description that you are currently attending a bachelor, master or doctorate.