To claim your Steam key (if you own 10YA on Desura), visit this URL:
http://www.desura.com/collection
And find 10YA on the list. Click keys. Your 10YA Steam key should be there.
To activate it on Steam, open Steam and press 'ADD A GAME...' and then 'Activate A Product On Steam'.
NOTE: The game will initially display in as 'unreleased' in your Steam library.
If it is not there give it some time to filter through the system
Enjoy!