Scroll to the bottom of the page and look at the lower left side where it shows your WoW Character's Face. Click the little "v" button that is shown to the right of your WoW character name. This MUST be done at the bottom left corner of the page. The v button at the top right corner will not work and will not change it.
It MUST be done at the BOTTOM LEFT CORNER OF THIS PAGE. Once you click the v button to the right of your WoW name, click the "Manage Characters +" Button, select your D3 profile, and then your D3 profile should now be selected instead of your WoW profile.
I had some problems after clicking the "v" button and after clicking "Manage Characters +" button. The drop down menu would just disappear. But if you click it again it will work and stay up.