Please make sure you have allocated 3GB-4GB to the launcher, and are running Java 8! If that does not resolve your issue, visit the forums at http://www.Galaxy-Craft.com and we will try to help you.
Type in the modpack name (Galaxy-Craft Lite) or paste the following url into the search box.
Finally, click Install at the bottom right of the launcher after you select Galaxy-Craft Lite from the list on the left. The launcher will handle everything else!