There is a FREE package for Painkiller, you can grab it here:
http://j.mp/pk_liteYou will also want to install the PK++ Mod which you can get here:
http://j.mp/pk-pp(PK++ Install Instructions:
http://j.mp/installpkpp)
Make sure you have the proper maps:
http://j.mp/pkmapsMap Pack:
http://j.mp/kothmapsDM_Sacred, DM_Psycho, DM_Absinthe, DM_Blink, DM_Fallen2, DM_1v1_Meatless, DM_Q1ZTNDM3BETA4, DM_5quid01, DM_ExMortis should be grabbed before the game, just drop them into your /Data/ folder!