You need to obtain a product key (free) and activate it. The
page you downloaded from has information about how to obtain the product key you need.
You may also want to read
this post in the FAQ section, about how to upgrade from the trial to the full version of the software. This is the only way to upgrade; you can't replace the trial product key with a full product key.
I'm not on the WHS team, I just post a lot. :)