It's not finding the FML classes it's looking for. This probably means one of the mods you added was made for a version of Forge sufficiently different from the one that comes with Infinity. Make sure you're running versions of those mods for Minecraft version 1.7.10. If you are, either update Forge (can't say I really know how to do that; you might be able to try copying over all the mods to a MultiMC instance and choosing which version of Forge to run?) or try slightly older versions of the mods you added.