Acrolinx is available as soon as you open a document, but you'll need to show the Sidebar before you can start checking.
To show the Sidebar, do the following:
Select Extensions in the Google Docs, Slides, or Sheets menu.
Select Acrolinx > Show Sidebar.
If you don’t see any Add-ons, it might be because you’re using Office editing. When you open Office files in Google Docs, Sheets, and Slides, you can edit the files but can’t use all the Google Workspace features. To use Acrolinx, convert Office files to a Google Doc, Sheet, or Slide. To get more information, go to Google Drive Help.