First, open your DAW and place the Spitfire Audio plugin on a MIDI channel.
Click the Unfold Device Parameters button (this is the small triangle next to where you would enable/disable the plugin). This will reveal the Configure option, which you must click to enable -the Configure button will turn green to indicate it is enabled.
While Configure is enabled, click on the parameters in the Spitfire plugin you want to map to your Push 2 and you will see them appear on your Push screen.
You can then right click the Spitfire plugin and choose 'Save as Default Configuration' and Ableton will remember these mappings each time you load the plugin.