Every copy of SoftDown theme need to be registred. Go to "My Orders" in your qalab account, find the activation code for the domain you are installing the theme on and copy it.
Next, in WordPress dashboard menu, select "SoftDown", enter your activation key and click "Activate".
Note that in case you want to use your theme on other domain name, you have to submit a ticket, and once you get a confirmation from us you can download the theme again and install it on the desired domain name.
Also note that running multiple websites on a single license is a copyright violation. In other words it is a serious legal offence and is strictly prohibited. Remember 1 site = 1 purchased license.