You can visit the hosting control panel (Virtualmin) for your site by putting "admin" infront of the main part of your domain name.
For example, if you had the domain name example.com, you could visit your hosting control panel at http://admin.example.com -- you should not include "www" anywhere in the address.
Note that your login details for the hosting control panel are different from the details you use to login to the customer portal. If you have forgotten your login details, visit your hosting control panel and click the link marked "Forgot your Virtualmin password?" on the login screen.