That's it !!! Big thank you w t ... it was due to that blank line between table and the line "#+TBLFM"
Thank you Nick Docos too for your reply :)

cheers

Matt

On Sat, May 8, 2010 at 5:48 AM, w t <1218121@gmail.com> wrote:
On Fri, May 7, 2010 at 7:21 PM, Matt Chastagnier <workandroll@gmail.com> wrote:
> Hi,
>
> I've just started with org-mode and have done several tutorials. But I'm
> stuck with this problem for 3 hours :
>
>
> I have :
>
> | item    | quantity |
> |---------+----------|
> | laptop  |        4 |
> | desktop |        7 |
> |---------+----------|
> | Total   |       11 |
>
> #+TBLFM: @4$2=vsum(@I..@II)
>
> I insert a new line [because I will want to add a new item] (M-S down) and
> update the table (C-u C-c C-c) :
>
> | item    | quantity |
> |---------+----------|
> | laptop  |        4 |
> | desktop |        7 |
> |         |       11 |
> |---------+----------|
> | Total   |       11 |
>
> #+TBLFM: @4$2=vsum(@I..@II)
>
> but my formula isn't updated automatically so the wrong field is updated. I
> would like the formula to become @5$2 instead of @4$2 automatically.

Before editing the table make sure there is no blank line between the
table and the line "#+TBLFM: @4$2=vsum(@I..@II)". Then use the
M-S-<down> command.