Have you updated the game to the latest version?

Need to know what version you bought, I am thinking it may be the original with no expansion packs.

Sins of a Solar Empire [Org stand alone]
Sins of a Solar Empire: Entrenchment [expansion]
Sins of a Solar Empire: Diplomacy [expansion]

All 3 above can be bought as a pack which is called "Trinity"

The newest release;

Sins of a Solar Empire: Rebellion [stand alone]

1st you will need to know where to install mods, then you can create a folder there which will be the new mod name.

Installing mods

Post back with more info.

EDIT: BTW, the new rebellion is on sale at steam for a couple more days, if you are serious about modding, I would get that.

http://store.steampowered.com/app/204880/

Well if the game did not come with the key you're basically out of luck, just like most modern computer games. In general buying modern PC games used is not a good idea.

That said the 1.05 patch for the original Sins of a Solar Empire was the last to be available on the internet even without the CD key, and it was the first version with mod support. However, only some very old mods will be around for version 1.05, none of the cool mods you might be hearing about now are even for the original Sins, let alone version 1.05.

Also on the steam summer sale going on right now you could by Rebellion, the latest version of Sins with all the expansion packs (3 of them) for $13 bucks, so I'm afraid you just got a very bad deal. The original should have sold for less than $5 even if it was new with the CD key. Take it back if you can.

Yeah, I don't think the Babylon 5 mod is for the original Sins. Mods are picky with the versions so always make sure you're getting the right thing.