[GRASSLIST:7553] 3D region in GRASS 6

[Claire Mautalen]

Hello Grass user, 
 
I would like to use v.vol.rst in Grass 6 but when I try to produce an elevation 
map, I have this following error message:  
Warning: there are points outside specified region--ignored 42 points (total 
points: 42) 
ERROR: zero points in the given region! 
I think I have to define a 3D region with g.region but I don't know how. 
If someone can help me?  
Thanks, Claire.