There is no mpoints.txt in NFSPS. Everything needs to be set in the model , and it's materials.
You need to create a cube in Zmodeler for creating a mountpoint ( it is convenient to create it where you want to see the flame ) , then you must assign this material to it :
" %_KIT00_BASE@Y=-90.0 " if you'll place all the exhausts ( stock , autosculpt , kits and bodykit ) at the same place , or as " %_KIT00_BUMPER_REAR@Y=-90.0 " thus making it dependent of the kit that will be installed.
EDIT : Read the tutorial please , it has a note about the effects such as the exhaust flames and such.
Tutorial : http://www.nfscars.net/forums/topics/nfs-prostreet-modding-tutorial/44218/1/#post-638853