Steam users typically manage DLC through the Steam client, though the EA app still runs in the background.
Installing varies slightly depending on your platform and launcher. The base game must be installed first (it is currently free on all platforms) before you can add expansion packs. PC & Mac (EA App) the-sims-4-island-living-installation-tutorial
If you have already purchased it, click the . If not, you must purchase it first. Steam users typically manage DLC through the Steam