The good news is that your local machine is now correctly resolving your domain name locally, so you've done all of that hostfile stuff correctly, and you can access the webserver on that machine, however, you haven't actually created any sites yet.
For the rest, some more background is needed.
Firstly, when you install ISPConfig, it installs it's own webserver, a completely separate Apache installation just for ISPConfig, that listens on port 81, rather than port 80.
The main Apache webserver installation is used only for the websites that you create using IspConfig.
In following the Perfect Setup guide, one of the things it told you to do (page 3) was to edit the hosts file and make it look like this (top section):
127.0.0.1 localhost.localdomain localhost
192.168.0.100 server1.example.com server1
and then to type
echo server1.example.com > /etc/hostname
Translating, this means edit the hosts file and put in the fully qualified domain name of your server i.e. server1.example.com.
Then edit the hostname file and put the same name in there.
then check it by typing
They should both produce the same result. If not, reboot. If still not, you have made an error in your editing, repeat until it works.
Obviously the name should reflect your domain and the name you have chosen for your server, and to repeat the server name here is server1. The fully qualified domain name of the server is server1.example.com
Once you have got that right, that is what you enter in the ISPConfig install. If it asks for a server name, enter it. If it asks for the fully qualified domain name of the server, enter it.
And to stress again, your domain name is not www.mydomain.com
It is mydomain.com. www.mydomain.com
is a subdomain of mydomain.com, that by convention and luck, may happen to contain a website.
You can check if Ispconfig is actually listening on port 81 by typing netstat -tap and looking for an entry for :81 and ispconfig_http. To access the ISPConfig admin path is e.g. IP is server IP
Note the https for secure site.
Final thought, under no circumstances should you call the machine www.mydomain.com
or anything like a url address. Name it after a dog or girlfriend or superhero, or stick with server1. The www bit is what gets hosted on the machine, not the machine itself.