Where are the Unity Launcher .desktop files?

Globally in

/usr/share/applications 

Locally in

~/.local/share/applications

If you want to add a custom launcher, create it in ~/.local/share/applications, make it executable, drag and drop it on the launcher*, and finally pin it (right-click on the launcher item → Keep In Launcher).

* Opening it using Nautilus doesn't seem to do the trick.

Unity 2D looks for .desktop files in the /usr/share/applications directory. This can be found by using the following in the Terminal:

locate firefox.desktop

For me it worked by adding one of the launchers that was already there (eg. gimp.desktop) using the following in the Terminal

gconf-editor

to add an entry to the favorites list under desktop/unity-2d/launcher.

cd ~/.local/share/applications is the 1st place that is look in.

Examples from my directory that I used to add quicklists:

~/.local/share/applications$ ls
banshee.desktop           gnome-terminal.desktop
chromium-browser.desktop  mimeapps.list
firefox.desktop           nautilus-home.desktop
gedit.desktop             ubuntu-software-center.desktop