I just tested just to be sure. When you choose expert mode, after you choose the partitioning scheme, you will get the screen where you get asked if you want to install headless or with graphical environment. If you want a graphical environment you should go ahead choosing 'NO'
I also tested default install. Then I just had the zentyal desktop right after installation. So IF you chose default install, and you don't get the zentyal desktop, there is something going wrong anyway.
Can you check the install log and see if there is anything that points to a problem?