I created a new domain in the DNS tab, after that i would like to create a website under the Sites tab, but can't find the newly added domain on the Domain drop down list on the "add new site" dialog. Earlyer I updated my system to Ubuntu 11.04 from 10.10.
I guess you accidently enabled the domain module without using it to add domains. Diasble the domain module under System > interface config.
You can try to reinstall ispconfig... but i'm not sure if will overwrite existing configuration. I will came back with an answer if i find any. Try to backup your actual installation and give a try, if the installation it's not OK roll back.
The installation is fine, plaese do not reinstall! He just has activated the domain module without using it to add domains, so the domain list is empty of yourse. The solution is to simply deactivate the domain module.