I had that issue when I was dealing with setting up my modpack. Let me go down a list of things that I did and tried. (yes mine works)
- Are you using Dropbox? (this will determine extra help provided if yes)
- Are you using WinRar? If so, I had a problem in which the archive .zip file wasn't being read correctly. I simply un-installed WinRar temporarily, reconstructed the .zip (just copied over the bin/config/coremods/mods folders) to a new zip and it was picked up.
- What Teraku asked, what version of Forge and did you compile the modpack.jar correctly?
- Did you make sure you can access the modpack when you put it on the internet? Either Dropbox, Google Drive, Mediafire, ad.fly? If there isn't a working link to your modpack, the launcher cannot download it.
Get back to me with answers and I'll help as much as I can.
Yeah, you forgot to ask more questions. You didn't ask enough!