I am happy that the "How to" is useful for you and of course for anybody else.
At the beginning of my "how to", i mentioned that i assume that your webs are stored under /var/www/.
This can be established, if you setup ISPConfig in "Expert" mode as descibed here: http://www.ispconfig.org/manual_installation.htm
I guess that you have setup ISPConfig in the "Default" mode, which means your webs are stored under /home/www instead of /var/www.
So, if you read /var in my "How to" it probably /home for you and it has nothing to do with symlinks.
Be sure that webmail.yourdomain.tld or better *.yourdomain.tld points to your webserver.
You can check this in the DNS-records of your DNS-provider.
I hope that i've answered your question now.