Acrolinx is available as soon as you open XMetaL Author, but you'll need to show the Sidebar before you can start checking.
To show the Sidebar, click Acrolinx in the top menu.
Click Show Sidebar.
The Sidebar loads in the resource manager on the left side of XMetaL Author. If you prefer, you can drag and drop the Sidebar to the right side of the screen. You can also adjust the pane to show the Sidebar in its fixed dimensions.