It's not for modding, it's for sharing files.
Just follow the instructions on the first post of this page:
http://forums.civfanatics.com/showthread.php?t=429816
Basically, it does the following:
You install tortoise.
You set up a folder called "Caveman2Cosmos" or something like this.
Then you rightclick this folder seleckting SVN checkout (as discribed on the post I linked).
Then the the Folder stars downloading Caveman 2 Cosmos. After this you need to rightclick it and select "Update SVN". Every time you do this, it changes files, deletes some or adds some so your folder will be at the very latest and most recent Revision. This is now what the beta version would be. Later, when V34 is released, you don't need to download the whole V34 (or later 35, 36...), just "SVN Update" your folder then
To play it, you need to copy/paste the folder at the correct place (BtS/mods...) of course.