Double-click the downloaded .zip. PIMS.app appears next to it.
Drag PIMS.app into your Applications folder.
Open PIMS from Applications and sign in with your Auriga email.
If macOS refuses to open it: right-click (or Control-click) PIMS.app → Open → Open. macOS asks once per install; after that it opens like any other app.
Picked the wrong version? No harm done — an Intel build won't open on an Apple Silicon Mac (and vice versa) but nothing else breaks. Just come back here and grab the other one.