I used this google.com/wave and wave.google.com and I got this page everytime:
I tried many times as I have used google wave just a day before. After repeated attempt, I thought there can be some problem. Then I understand that the Google account I am already logged in has no google wave account associated. That has the sandbox account
associated with it. Sandbox account has this url:
So, I logged out from there and I got the login page. From there on I logged in to the account where I have preview account. The Sandbox one is Developer sandbox account.
On another account I have invite power also, which I got few days back. So, I am going to invite myself for another Google account.