Gorah he is talking about the giant button right in the middle of SimCity 4 main menu, that launches a WebBrowser and enters SimCity.com site.
Unfortunately, it doesn't work, in fact it don't worked even when the game launched, they changed the site only mere months after the initial launch.
So just ignore that button. Also the in-game mod download system that was planned never worked anyway, and noone is willing to use their own servers to host regions (the game has a region download system) because they are too big.
Just visit Simtropolis or SC4Devotion and get the mods and maps there (and again, you can't get pre-built regions, only terrain maps to import).
