Generally speaking, discounts are applied automatically if you are logged in. In fact, you must be logged in to see and get member discounts. Automatic discounts appear on the page, showing both the non-member price (with a line through it) and your member’s discounted price. Other  than logging in, there is nothing special you need to do. The discount will be applied during checkout.
Sometimes, the discount is in the form of a coupon or code. In that case, you’ll find that listed as a separate entry in the “Members-only” menu (the one you used to get here.) Note: you must be logged in to access that as well.