I am new to HOI4 modding.com and i am not sure how to add states. Do i add them in game or throught the mod files with state ID's?

When editing your country, you will need to load the map by pressing this button in the sidebar:

This will add a "States" item the the sidebar menu, and in there you can click a state and say whether your country controls it, has a core on it, or both.

Just note with your mod, the "Country Overhaul" option which you have checked means it will remove every country from the game, apart from the ones you add. If this isn't what you're intending, you will want to disable that before exporting your mod.

Thank you.