On the show's page you will see a button (actually two buttons - the little one just to the right with an image of 3 bullets allows you to add the show to one or more of your custom lists) below the show's picture that says +Add to List. If you click it open you change the status to Currently watching, Completed, Plan to watch, On-hold, Dropped or Not interested. You can change the number of episodes watched, rate the show or leave notes.
On the left of that same General box you change to the Advanced screen or view History.
Many things you do in this part of the application will change your watchlist.
I hope this helps. Maybe we should ask MDL to work on a user guide?