To Add/Remove items from your Watchlist you first need to be logged in. If you don't have an Account, you can create a Free Demo Account
To add a contract to your watchlist, go to the contract (find it in search, browse for it...) and in the Contract screen you will see a +WL in the menu. Clicking on it will "Add to Watchlist". Now you will see -WL instead. Clicking on it will remove from Watchlist.
You will also be able to Remove items by going to que Watchlist Section in Quotes. You will see the -WL link next to each item.