Hit F8 while airborne, then press space when you're over the target duck.

Hi wessel. Strictly speaking you don't have to use parachute after the, er, garbling hook... If you are precise enough, after bouncing off the first duck you can just press [Space] to fire the hook again, and (if successful) grab onto the second duck. The parachute crate is just there to make the mission easier.

Alternatively, if you would like to use the parachute in rapid succession, you will need to do as claymore suggests and rapidly change to that weapon. If your F1-F10 keys don't work (mac-user, eh?) then I would suggest changing your controls. This can be done in the following way:

1) Go into the team creation menu.
2) Click the edit button after selecting the team you play with.
3) Click the "advanced" tab
4) A list of the "Basic controls" should come up. Change from this to "Weapon controls".
5) Change the slot keys (by default F1-F10) to something you can use.
6) ...
7) Profit??

Note: After rapidly changing from Rope to Parachute, you may have to push fire twice to activate the parachute. Also you can steer the parachute using the arrow keys, but you probably already knew that.

Good luck.