How can one gain the Dota 2 achievements?
Solution 1:
There is no way to unlock achievements in Dota 2 as of now.
What you see are just placeholders, they've been there for a long time but they don't really exist in-game.
Response from Valve employee on the question (29 May 2013 3:34am):
Dota 2 does not currently have achievements.