Where can I find Altair?

Solution 1:

This is located in La Valette's castle, very early in the prologue (The King's Will, Part 2 - Barricade).

The king orders you to capture and use the ballista. You drop down the scaffolding on the wall and turn left to see the ballista. You'll have to fight several foes to reach it. Once you've beaten them, continue past the ballista, toward the closed gate. Poor Altair is on your right, slightly hidden by the castle wall.

Upon finding him, you'll receive a bonus ability, assassin, which grants you +25% damage when attacking an enemy from behind.

Solution 2:

The video has the mission info in the top-right corner. This corresponds to a part of the prologue, not the Chapter V Barricade quest as I previously thought.

When you're in the courtyard trying to take the ballista, run past the ballista towards the gate you can see people fighting behind. Then hang a right and run into the corner, there's a white robed corpse laying there that Geralt will comment on. Some how this teaches him how to do 25% more damage when he attacks from behind.

Source: http://www.gamefaqs.com/boards/975399-the-witcher-2-assassins-of-kings/59161336