1. LOGIN TO GOOGLE APPS
You have to be the administrator for your Google Apps domain in order to enable Google Wave.
2. CLICK "ADD MORE SERVICES"
Next to the "Service settings" heading there is a "Add more services" link. Follow that link:
3. CLICK "ADD IT NOW"
On the next screen, you just select the "Add it now" button:
4. CONFIRM YOU REALLY WANT TO DO THIS
Google throws up a page just warning you that you can't get support for Wave through existing support contracts... and other warnings. Just hit the "Yes, enable Google Wave" button:
5. START ENJOYING WAVE WITHIN YOUR APPS DOMAIN
That's it. Now you can just direct people within your Google Apps domain to go to:
and they can start waving! Here's what it looked like for me:
Now, as noted in the warnings, users in your Apps domain can by default also invite external people into your corporate waves. That may be a great thing... that may be something you want to think about.
In any event, enjoy waving within Google Apps!