To enable techies, first navigate to the npc folder within the Dota folder:
C:\Program Files (x86)\Steam\SteamApps\common\dota 2 beta\dota\scripts\npc
If you can't find an npc folder, create one inside the "scripts" folder.
Paste this file into that folder: activelist.txt
Finally, go to your Steam library and right click DOTA 2, click "Properties", "General" tab, "Set Launch Options" and add this line in there:
-override_vpk
You'll now be able to play Techies in bot matches and Local Lobbies only.
NB. They will appear as a selection in normal matches but you won't be able to choose them.