Using Host Headers in IIS7 / Why is the host headers text box disabled?

If you want to run two sites on the same port you will need a Wildcard Certificate. Eath site will be assigned a different host header (= URL) in IIS (e.g. site1.yourdomain.com and site2.yourdomain.com).

What if the host name text box remains disabled although I have a Wildcard certificate?

The friendly name of the Wildcard certificate needs to start with an "*" (e.g. *.yourdomain.com) as you can read here.

You can still use the command line though to set the host headers appropriately.