The Atlanta CreeperHost network seems to be down at the moment (has been for the past ~12 hours), but the England mirrors work fine.
Once you get the launcher installed, you'll likely still have trouble downloading packs, since it tries to get them from the same place, but we can fix that too.
Here are some links to the FTB Launcher on the England 2 server, which seems to be fully functional right now:
Launcher EXE ,
Launcher Jar
Once you get the launcher running, just open the options tab, press the advanced options button, and change the Download Location from Automatic to England 2. It should then be able to download packs and install them properly.