You can use Acrolinx in different views in AEM. To know exactly where to find the Sidebar, refer to the view you're using:
Touch UI
To show the Sidebar, toggle the side panel and select Acrolinx from the menu.
Content Fragment
To show the Sidebar, toggle the side panel and select Acrolinx from the menu.
AEM Guides
To show the Sidebar, toggle the side panel and select Acrolinx from the menu.