I have posted the question yesterday, then I was able to figure it out by myself. As I haven't found any complete guide, I'm writing down the steps just in case.
So the question is: "How do I install db2 express c on Mac OS X?".
First of all you need to edit/create the /etc/sysctl.conf and insert the following:
reboot. Then google db2 express c mac osx and download db2 express c 9.5.2 for mac osx. Once downloaded uncompress the file
enter the exp directory and start the installation wizard as root
before continuing you need to create 2 users, one for administration purposes and another one as instance owner. For the second user name use no more than 8 characters and use just letters, otherwise the node name won't be accepted later on. Follow the installation wizard steps: enter the username for the DAS and for the instance owner. Once the installation is complete, you need to switch to the instance owner (dbuser in our case):
add the following line to your .bashrc file:
exit and relogin. If everything went well you should be able to start db2 (as a root) and create the SAMPLE database