The theme comes with its own popup login form. You can enable this from Aardvark > Theme Options > Membership > Frontend Popup Form.
If you are using Youzify and have their membership system enabled, the theme's login form is disabled as the two membership systems conflict.
You can disable Youzify membership system from Youzify Panel > General Settings > Activate Membership System.
If you want to use Youzify membership system you can enable their popup login form from Youzify > Membership Settings > Login Settings > Enable Login Popup.