Forge does not work for 1.6.4. After the Heartbleed incident, the hashes for the files used by 1.6.4 Forge changed from what I've seen, which, long story short, broke Forge/MCP as it depended on these hashes not changing. Update to 1.7.10. You should have been using it anyways, why you're using 1.6.4 is beyond me. Most of the major mods have updated, so that's not a valid excuse for using 1.6.4. Plus, getting to 1.7.10 will make updating to 1.8 much easier when Forge is released for it (which won't be too much longer, I'd assume, 1.8 MCP is already in pre-release). 1.6.4 is in the past. Let's keep it that way.