Head over to the Enterprise Settings tab and look under the Account Information section. There you should find a button marked Add-on Products.
You will arrive on the Add-ons page where you'll be able to select and add eligible products, then click continue.
Once you have selected the add-ons you'd like to purchase, finish your purchase by confirming payment with your existing payment method or update with a new payment method. Click "Complete Upgrade" to finish your purchase.