Garageband versions 10.0.0 and all updates of 10
In Garageband the first you will want to do is add an Audio, Instrument or Drummer Track. To access the Plugins menu SMART CONTROLS must be enabled in the project.
This will automatically open the Smart Control menu in the bottom section of your Garageband Project. If you would like to toggle this view between SHOW/HIDE you can use the Key Command "B" on you Mac's keyboard or perform this under the drop down menu.
Click inside the Audio FX Box and this will automatically open Garageband's contextual menu. Scroll down to AUDIO UNITS and find and select your Focusrite & Novation plugins from here.