I'm confused what is "saying Mac". You're looking at http://eternal.bungie.org/
right? On the left should be one link for Eternal itself, which should work for all platforms, and three links to the automatic current download pages for the Mac, Windows, and Linux version of Aleph One. Which you click the Windows version link, are you getting something for Mac?
I just tested (on a Mac) and I
get a Mac version. I don't know why that's happening; one of the Aleph One devs here can maybe clarify? (Is this just some user-agent-detection going on and redirecting me to the Mac version, or should I update the links on the site to something else?)
In the meantime, 7homas, there's also a link at the bottom to http://source.bungie.org/
where you can definitely find the current versions of Aleph One for all platforms; it looks like the link you want specifically is https://github.com/Aleph-One-Marathon/a ... 20-Win.zip
in case you have trouble finding it.
ETA: I just updated the website to have a single download link to the standalone Aleph One app downloads page on lh'owon.org, and updated the main Marathon Open Source link to the new domain too.