According to your website Avast! offers Mac Antivirus for free: http://www.avast.com/free-antivirus-mac
Soon my license will expire. According to the built in subscription page Avast! is running in trial mode now (14 days left).
The only button visible is “Insert license file”. There is no option available to renew my free license for another year.
Problem solved: I have downloaded my version of Avast! Mac Antivirus through Cnet some time ago. They offered the wrong edition, i.e., the Pro edition instead of the Free edition. The difference between these editions is explained here: http://www.avast.com/en-us/faq.php?article=AVKB72#idt_07
I have uninstalled the Pro edition, and installed a fresh copy of the Free edition instead.
In the subscription screen the “Register now” button is visible again.
From the above it seems there is no way to use Avast unless you purchase it over the internet that is something I would rather not do. I am using the free edition and it worked a treat the other day when my W7 crashed. It did a complete scan that took over an hour and now everything is working just fine. I have a free trial for better cover and will seriously consider upgrading but a CD to do that would have been preferred.
All posts were helpful. For me, If the antivirus is not paid, I feel that it is not going to work well or going to expire after a few days without any prior notification! :-\