Your game directory is the directory you specify in your profile.
fml-client-latest.log is a file inside the logs folder, which inside your game directory.
@Quizer9O8 This has nothing to do with the original post. You should make your own thread next time. As for your crash: it's an issue caused by Optifine. Remove it and report it to the author.
Post the FML log (logs/fml-client-latest.log inside your game directory).
Also, remove every mod you have. If it stops crashing, it's one of the mods' fault.
Why 1.8.8 out of all the 1.8 versions? You should really update to 1.11.2, or soon to 1.12. If you must use 1.8, use 1.8.9.
Do you have Optifine installed? If so, remove it. If the crashing stops, it's an issue with Optifine and you should report it to the author. If it doesn't, post the new FML log without any mods.
You're using the wrong Register class (Tcp.Register), while you should be using RegistryEvent.Register.
Also, because the RegistryEvents are called before preInit, you should annotate the class with @EventBusSubscriber, so the class actually gets registered.