Download the current WIP is a not very obvious, multistep process. It should be simpler.
Go to the McNeel Downloads page:
Click on “Serengeti build”
This takes you to a forum post by Bob:
Click on " Details and download" which does not take you to the download page but rather to a forum post by Brian on a different thread:
Clicking on “Download now” takes you to:
Clicking to download on this page actually results in the latest public build of the Rhino 9 WIP being downloaded.
The website Download page and Bob’s post should also lead directly to this page.