Before you can post here at the forums, make sure that you have a MySteinberg user account. Once you have created your MySteinberg user account, you can extend it by creating a forum profile. If you already have an account, creating your forum profile is all you need to do. The additional forum profile allows you to actively participate in the forums and post messages depending on the product licenses registered in your account.
Once the profile is set up, you can either access the forum through MySteinberg -> Forum or just by using the URL
http://www.steinberg.net/forum and log-in with your MySteinberg user name and password.
More details about creating an user account can be found here
Please also have a look at the forum FAQs.