When you say 'it won't open' what do you mean? Does the game open at all? Does do you get errors? Do you get in, but see no new leaders?
We'll need specific details in order to help out. Try an action-result format: When I do this, then this happens. Also, compare what you expect to see versus what you expect to see.
In general, make sure you are copying files to the right places (it should match the structure of the main assets folder), make sure all of your files and folders have the right name (my first time modding, I spent a day banging my head on the keyboard because I'd misspelled 'Civilizations'). Make sure that all of the references you make in the XML exist in all of the correct XML files (for example, you need to assign leaders to civs, if you use TXT_KEY_ tags, make sure they exist in the correct XML files, make sure the tags are the same (I always mix up _ and - for example).