OK, I'll explain this in fine detail. I apologise in advance if this seems at all patronising.
What I have linked again here is the IC2 repository. This is a Jenkins build repository which keeps copies of every build published to it and records the changes made in each version. It also includes builds that are broken or buggy, because this repo is mainly intended for IC2 developers adding changes for each other to examine. Faulty builds are marked with a red dot, working builds are marked blue. There is a marker for recommended versions of code (i.e. stable releases) but the IC2 devs don't use it. Note that I keep saying "builds", not "releases", because these are developer versions not verified to work properly by the team. IC2's last official release was for Minecraft 1.4.5 so the FTB team and you have no choice but to use a developer version to stay in date. Occasionally there's a really silly bug that creeps into a build but most of the time they are relatively stable and playable.
To download the version you want, you click by the build number you want. For example, IC2 version 1.115.
207 is build #207. As you can see, there have been many versions built since then, so you can choose to use a newer version instead. All that is required is that you, the server and anyone else playing on the server are all using the same version of the mod, otherwise you will not be able to connect to the server to avoid major crashing problems server-side.
Download the version you want to use - make sure you download the primary download and not the API as that's for people making mods - and add it to your .minecraft/mods folder. You will need to remove any duplicate versions of IC2 you have or the game/server will not start - this applies to all mods.
It should work fine from there. Hope that helps.