thanks for you reply.
I was following step by step installation documentation in ispconfig3_install/docs/INSTALL_DEBIAN_5.0.txt (except for omitting the quota settings but I don't think this should have influence on postfix configuration).
It is needed that you enter the install folder with cd and then execute php install.php!
I did run it exactly this way.
I wonder why are the folders like ispconfig3_install/config/postfix/ even there if no files belongs to them? Why was I getting errors during installation that PHP is unable to find file and copy it to /etc/postfix/main.cf? (Unfortunatelly I didn't save the messages. But I can run the installation again and see if I can reproduce the problem.) And why postfix is not able to run with "fatal: open /etc/postfix/main.cf: No such file or directory" error message?
I'll try to find out what I did wrong. So please take my post just like information.