Just installed on 10.10.5 and it opened right up.
The dev’s website is active, so this app can still be downloaded, and additional screenshots can be seen on the website.
BE SURE TO GO TO THE WEBSITE BECAUSE THERE’S A LOT OF INFO THERE ABOUT THE OPTIONS AND PREFERENCES!
It even tells you how to send a Xmas message from within the app via Apple mail.
http://www.freerangemac.com/TreetopLights.html