Open OOTP, on the splash screen click on "settings" at the bottom. In the settings box click on the "troubleshooting" > "open folder containing customizable data". That will open the main data folder in a window on your desktop taking you to the file location you are seeking.
My advice is when working in these files is you go back and close OOTP. IE bring OOTP splash screen backup, then in the settings box hit cancel, and then exit on the bottom of the splash screen. This will make sure all files "stick" once edited and they will load properly when you reload OOTP. While this isn't required for many of the things you mod there will be a few files that will cause problems in some areas. It's just easiest to close the game, make the edits, reopen the game and not have to worry about whether something is going to work or not.
To avoid the steps above once the folder is open you can create a shortcut to it. Now you have instant access any time you want to mod something.