By default, pressing the short-cut 6 to open up the Particle View requires you to have the Keyboard Shortcut Override toggle button turned on.
In order for it to work even with the Override button turned off, you will need to run this short max script:
macroScript Open_Pview category:"Particle Flow" ButtonText:"Open Particle View" tooltip:"Open Particle View" ( actionMan.executeAction 135018554 "32771" )
This will create a macroscript which can be found inside your Customize User Interface dialog, under the Particle Flow category, named Open Particle View. Simply assign it to the short-cut 6. It’s a toggle so you can press the short-cut to turn it on or off.