From what you are saying I believe you already registered, just weren't able to get back to finish the purchase.
Try the following:
Go to https://vponline.celeris.com/auth/login
and hit the 'Forgot password?' button.
Enter your email adress and Submit.
The retrieve email function was just fixed, so you should get an email from Celeris with a link.
If you get one:
Follow the link and change the password
Use your email address and the password to log in
Click on 'Store' at the top. Now you can add Virtual Pool 4 to your cart and check out.
(only) If you don't get an email with your password:
Open Internet explorer
Goto 'Extras-> Internetoptionen'
In Tab 'Allgemein' click button 'Löschen'
A new window opens. Click checkbox 'Verlauf' and button 'Löschen'.
Now go to http://vponline.celeris.com/vp4offprod
You should see a 'Buy' button there.. use it and start the purchasing process.