1) i work only with pdflatex sometimes with TexNiscope, sometimes
with Acrobat and sometimes PDFView

That's fine. The current version uses pdflatex by default, and you can use TeXniscope or PDFView as the viewer.

Acrobat is not so useful, because it can neither auto-update nor be signalled to update. Similarly Preview.

actually : TM_LATEX_VIEWER

1) When i use LaTeX Watch with this little code
I have an error :Latex Watch Error : Failed to process preamble

That's interesting. I don't get an error when I try it. If you look at the log file directly, what do you see? (The log file is named .essai.log, assuming your file is named essai.tex. Note the dot at the beginning.)

And then i have a error when i want to watch the .log

Ah! I bet that's because you don't have "mate" in your PATH.
Thanks for the report: I'll add its default location to the script.

2) Why Gv and not Acrobat ?

There seems to be some confusion here. Gv is only used in PS mode, it's not used in the default pdflatex mode. As to why not Acrobat, I explained that above.

ps : dvi --> ps --> pdf in not the better way to obtain a pdf !

Of course not, not generally. But if you have PostScript specials in the DVI, then it's the only way to produce a correct PDF.

Robin