It's there - you just have to go to Add Services. It's available on free too.

I tried browsing to https://www.google.com/a/mydomain.com/ on my domain that has Apps Standard. I get:

>> Service not available

>> This service is not enabled for mydomain.com. If you believe this is an error, please contact your domain administrator or try again with the following link.

>> http://wave.google.com/a/mydomain.com

I don't see anywhere in the Dashboard to enable it, either. Bummed.

Try going to the "Add more services" link.

It's on the main Google Apps domain management page, right next to the "Service Settings" header (the header of the sections which has things like mail, calendar, etc.).

Worked for me, and I'm on the Standard Edition.