I've looked high and low but can't find a keyboard shortcut to select an app from Mission Control.
I want to press F3, then "some keyboard key(s) and not have to click on the trackpad" to select the app I want.
The best alternative solution:
Press command-space to open spotlight, then start typing the title of the app (or whatever) you want and press enter.
There is free app called Karabiner that allows you to completely customize your keyboard short cuts on OS X including assigning "Launcher" keys combinations to launch apps.
Full disclaimer: I have used Karabiner, but I prefer to just use Spotlight (Cmd+Spacebar) to find the app I am looking for; much less to remember.