When pressing the super button the dash opens. It will always open in the home format. On the bottom you can choose between different lenses, the second one being the application lens with filter functionality. Is there a possibility to set the application lens as the default when opening the dash?
You can also use shortcuts:
Super + F = File lens
Super + A = Application lens
Super + M = Music lens
Unfortunately this feature has not yet been added to Unity, but it should be there in 12.04.
You can track this bug as mentioned by doug in the comments:
https://bugs.launchpad.net/ubuntu/+source/unity/+bug/885738