When you have purchased a package that includes Dropmysite or have purchased the add-on separately, you should firstly navigate to the subscription itself using the drop down menu top right of your control panel:
Once on the required subscription click the 'Dropmysite' link which will take you to the overview page of the package.
This page will show you how much data your package currently has and how much of that allowance you are using. From this page you can add Databases, Websites, upgrade your allowance and most importantly log in to the full 'Dropmysite' control panel where you can fully configure your service. Please see configuring Dropmysite for more information on this.