With the Installers tab open, and a page with downloaded mod as a zipped package open, You just left click on the zipped mod and then move it over the Installers tab (drag), then unclick it (drop). You wait for it to be installed(may take a few seconds) then you can highlight the mod with your mouse and move it to where you would like to place it in your Install Order. Newly installed mods usually arefound to the bottom of the Installers tab.
Once this is done, then you can use the Wizard to install the mod into your Data Folder.
EDIT, Take a look at my signature.
TESCOSI and Alt3rn1ties Pictorial Guide have better instructions than my typing.