I am assuming you want to stream music and video from your Raspberry Pi to your Xbox 360. What you are trying to do isn't file sharing in this case. You are attempting to stream content. This typically requires using DLNA to get this content stream-able.
The best program that I've found for streaming to an XBox 360 on a Linux based embedded system is MiniDLNA. The ArchLinux website has a great page for miniDLNA that should help in adding it to your system. I've had limited success at using MediaTomb with my embedded systems but some folks have had luck with it.
My system is a Seagate BlackArmor NAS and the miniDLNA worked great for my configuration. MediaTomb had more features but overwhelmed the CPU for my system.
I would be interested in hearing more about your setup and what you are trying to do if this answer doesn't help.