Adding a stock to your watchlist is very simple. Simply search for a stock (or click on one that you already own). In the top right-hand corner of the stock detail screen, you should see a "Add to Watchlist" button. Once you click on that, the stock will now show up in your Watchlist tab.

Alternatively, if you want to remove this stock from your watchlist, you can either un-click that button on the stock detail page, or you can click on the "Star" next to the stock listed on your Watchlist tab.