When starting a new game, the first pop-up screen should ask you which SI database you want to use (e.g. default or 13.3.0. update). At the bottom of that pop-up there is a box you can tick or un-tick next to Editor Data Files. Tick that box, click ok, and the next screen should be the option to tick whatever lower league database you've downloaded, assuming you've placed them in the right folder.