From Main Menu, click Dashboard.
Click MARGIN at the top right of the page to switch to margin market.
Also at the top right of the page, click Add Product.
Click on the product you want to trade to add it to your dashboard. Note that this is different than adding a currency. In margin trading, when you add another product, you are still trading with the same currency, just betting on the price of another product normally not selectable in Spot.