For those still unsure, this issue is caused by the host new.creeperrepo.net being unavailable to many users around the world. This is a CreeperHost problem outside of FTB's control, and is being worked on, but seeing as this is the third or fourth time we've seen this issue in as many months, i've knocked up a simple work-around.
Firstly its a good idea to make sure you are actually experiencing this particular issue. This part is easy, just visit
this link, and see if you get an error. If you do, read on.
For those who know what it means, all you need to is add a hosts entry for new.creeperrepo.net to point to 192.111.138.82 which is the address of the Atlanta mirror.
For those who glossed over for that part, and are running Windows, I've made a simple script that will make that change for you.
https://www.dropbox.com/s/tcjiwf7xg8j7ai6/CreeperRepoAtlanta.cmd
Just run the script and it will guide you the rest of the way.
Please remember to run this script again in a few days to undo this fix, answering No to the question it asks, or you may encounter issues later down the track when/if the Atlanta server goes offline.