IsGrenInNoGren is used to see if a grenade is in a "nogrens" area. By default, a trigger_ff_script named "nogrens" will not allow grenades to explode while the grenade is touching the "nogrens" trigger_ff_script entity.
IsGrenInNoGren( ent_id )
The ent_id passed in is simply an integer index that refers to an entities game code ENTINDEX().
If you want trigger's to react to grenades you MUST 'check'/'tick' the "everything" option on the flags tab of the entity in Hammer.