Here is a working link: Dota 2 Survey
*Make sure you have Steam running first and that you have updated to the most recent client version.
The email from Valve would have been something like:
You recently indicated you want to play Dota2. Before we send the first batch of invites we need to collect a little more information from you about your level of gaming experience and your gaming rig.
To begin the survey go to the machine on which you intend to play Dota2, start Steam and click this link: steam://takesurvey/1/ (if you haven't restarted your Steam client for a few days, you might need to do that before clicking that link).
To install Steam go to http://www.steampowered.com/about
The Dota2 Team