Install db2 express c on Mac OSX 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:

 kern.sysv.shmmax=1073741824                                                                                                                                                 
 kern.sysv.shmmin=1
 kern.sysv.shmmni=4096
 kern.sysv.shmseg=32
 kern.sysv.shmall=1179648
 kern.maxfilesperproc=65536
 kern.maxfiles=65536


reboot. Then google db2 express c mac osx and download db2 express c 9.5.2 for mac osx. Once downloaded uncompress the file

tar -xzvf db2exc_952_MAC_x86_64.tar.gz


enter the exp directory and start the installation wizard as root

cd exp
sudo ./db2setup


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):

su dbuser


add the following line to your .bashrc file:

. ~/sqllib/db2profile


exit and relogin. If everything went well you should be able to start db2 (as a root) and create the SAMPLE database

sudo db2start
db2sampl 
db2


That's it!