You can purchase WX5 v15 start here

www.websitex5.com

If you have already purchased and registered WX5 start, you can click on your profile (top right) and under my licenses you can retrieve your licence/activation key.

Hi Kiet, 

I can see you got a WebSite X5 Start licence yesterday, you will find it in the "My licences" section.

Please let me know if I can be of further help. Kind regards.