Check this page, just make sure you get the version of the manual that is applicable to your version of ACL. Unfortunately, it appears the page has not been updated for some time so the manuals for the most recent versions are not there. 😢
I have a more recent version, 22.2.0.53 and presume that nothing has changed. Go to the top menu, Help, Open manual, and it should open. Note the location of the file. I don't recall having to download this separately; it was likely installed with the program. F1 should also give you the Windows-style Help screen.