I had this issue for a while on my Mac Book Air where there would constantly be a progress bar below the Launch Pad icon.
If you are still experiencing issues with your download, please go here: https://answers.unrealengine.com/questions/24311/read-this-if-your-download-has-stopped-around-81This topic is now being closed.
It is recommended that you uninstall them, reboot the machine, and then re-launch the game.
These can also effect game performance and hinder the connection to the game servers.
Most of our data is chunked, and is usually under 1MB so that's ok even if you have a really slow connection, however at this point in the download it's attempting to download a 50MB file.
The fix will be in a future version of the launcher, but for now, you can edit Default to increase the timeout value.