How can I launch MATLAB 2012?

Try navigating to bin folder in your Matlab installation directory:

cd /usr/local/MATLAB/R2012a/bin

Then execute Matlab from there:

./matlab

Source: How do I launch MATLAB on Linux?

If that worked for you, you can create alias:

cd /usr/local/bin/
sudo ln -s /usr/local/MATLAB/R2012a/bin/matlab matlab

This trick will allow you to launch matlab from any directory, as you were trying to do. Note that you should use sudo for the last command, since it creates alias globally, for all users.

If you are not in sudoers list (i.e. you cannot use sudo command), you can create alias locally. To do that, open .bashrc file, which is in your home directory:

gedit ~/.bashrc

Add the following line to the end of that file:

alias matlab="/usr/local/MATLAB/R2012a/bin/matlab"

Finally, reopen your terminal, or type source ~/.bashrc and execute this command.

Source: How do I create shortcuts, aliases, or links to MATLAB?

I've just installed Matlab 2012b on Ubuntu 12.10 and followed up by installing the matlab-support package in the Ubuntu Software Centre:

sudo apt-get install matlab-support

(As per this answer in a related question: https://askubuntu.com/a/140078/16835)