You need to register in order to download the latest Altiverb installers (both mac and windows), please go to:
www.audioease.com
and fill in your e-mail address (the one you'd like to register for Altiverb). After this, you'll get to a registration page; fill it in and hit proceed. After this you can access the download at:
www.audioease.com
Fill in your email address right below where it says: UPDATES. Then hit the button and download the right installer.

Altiverb also needs acoustic samples, you can pick up individual sets from:
www.audioease.com
There is also one big (800 MB) download (including both Mac and Windows installer) at:
www.audioease.com