[AUCTeX] Tex-command-master not found!

[Z]

Hello everyone, 

I am a newbie to Auctex. I am using Carbon Emacs + Auctex under Mac OS X. I have two problems. Can anyone 
help me out pls? Thank you very much!

Pb. 1 
M-x C-h c C-c C-c      says C-c C-c is short for Tex-command-master 

and C-c C-c            does invokes (Latex) but contradictorily, 
M-x Tex-command-master does NOT match to any command!!


Pb2. the following does not work when I tape C-c C-c RET
from _part.tex. Thank you for your ideas. 

-----------------Here is the source file _main.tex----------------
\documentclass{article}

\begin{document}
Hello this is the main part.
\input{_part}
\end{document}

-------------- Here is the source file _part.tex------------------

Hello

%%% Local Variables:
%%% TeX-master: "_main"
%%% End: