Hallo
Ok the problem is
you cant sett NULL of this function.

pXent_setcollisionflag(eBlob, NULL, NX_NOTIFY_ON_START_TOUCH);

you must realy set the second entitie zb. bump of your terrain.

you = ent_for_name ("terrain_001"); // get pointer to that entity
pXent_setcollisionflag(my,you,NX_NOTIFY_ON_END_TOUCH);
my.event = Plop;

Then its work fine. ;-)

Tele