Miguel,
1) First you need to install correctly MacTeX When MacTeX is installed, you have in the pref system a tool : "Distribution TeX". With this tool you can verify what are the versions of macTeX installed in your system.
2) You can verify in the terminal with $ echo $PATH for me the PATH value is
[ego at mbp:~] $ echo $PATH /usr/bin:/bin:/usr/sbin:/sbin:/usr/local/bin:/usr/texbin:/usr/X11/bin
Actually you can install the MacTeX 2011 here : http://tug.org/texlive/mirmon/
Is a prerelease but it works very well. I think if you install a new version of MacTeX the past would be correct. The path of the new version will be :
/usr/local/texlive/2011 you can verify actually if you have /usr/local/texlive/2009 or 2010 ?
I think that your problem comes from the installation.
Logically with "Distribution TeX" you can see the new version of MacTeX
3) you can modify directly the path in your system. I'm not a great unix expert but I think you must have in the /etc/
a file named "paths" that contains
/usr/bin /bin /usr/sbin /sbin /usr/local/bin
and a folder "pathhs.d" that contains a file named "TeX" . In this file you need to have /usr/texbin
I hope all these informations can help you
Greetings
Alain