Did you edit the modpack in the launcher by removing any mods? That error means you are missing COFH. That is the mod that makes Minefactory Reloaded and Thermal Expansion work as well as controlling ore generation in the world. Make sure it isn't in the disabled list under the Edit modpack button in the launcher. If you haven't touched anything there, then your download must have gotten corrupted or not finished properly. If that's the case, you just need to redownload the pack.
