@BitterSweet-xo The store can be a bit finicky in macOS, but it almost always works for me. Try using Firefox, which is more reliable than Safari or Chrome. Open the launcher before downloading. When you click Download, you should get a pop-up that asks what you want to use to "open" the download. Choose "The Sims3" with the green plumbob icon next to it, and click Open Link. If the base game isn't among the choices, select Choose other Application, and choose the base game icon from the list.
Please also make sure Firefox isn't set to block pop-ups entirely. Depending on your settings, it can give you a warning (a yellow banner across the top of the window) that says it blocked a pop-up from the site, or you may not see that at all. Either whitelist the store, or temporarily allow pop-ups while you're downloading content.
