[metapost] Problem with metapost and stacksize

[Olivier.Jutand at ac-orleans-tours.fr]

Hello,

I'm a french maths teacher and I've been using LaTeX for several years and Metapost for my diagrams. But I'm afraid I face a problem I can't solve alone.
I wanted to set up Ocaml and camllight on my Mac (OS 10.5.8).
It was asked to change the stacksize
I copy those lines in the Terminal 

echo 'limit stacksize 64M' >>~/.tcshrc
echo 'setenv PATH "${PATH}:/usr/local/bin"' >>~/.tcshrc 
/usr/local/bin/ocaml
echo 'limit stacksize 64M' >>~/.tcshrc

or

echo "source .bashrc" >> ~/.bash_profile
echo "export PATH=${PATH}:/usr/local/bin" >> ~/.bashrc
echo "ulimit -s 64000" >> ~/.bashrc
echo 'limit stacksize 64M' >>~/.bashrc

But now if I want to compute a simple diagram with metapost in Texshop, 
nothing :

"limit: stacksize: Can't set limit (Operation not permitted)" in the Console


If any of you could help me ...

Thank you !