Printscreen isn't like the modifier keys, Flirc already understands it. If it's not shown in the full keyboard layout on your OS, try programming it from the command line. On a Linux box I use:
flirc record printscreen
On Windows it should work with:
flirc.exe record printscreen
I'm sure there's a way to program from the CLI on OS X too but not finding the location of the flirc command at the moment...