There seems to have been some confusion over this, possibly because its a different system than that used for The Sims 2, so I've decided to post a sticky explaining how it works. Hopefully, it'll save me a few individual posts, because I'm just lazy like that (or maybe just too busy playing... I mean working... on Sims 3).
So, here's the step by step process for those of you who have been having trouble, starting with registering your game to your account and making sure that The Sims 3 and all related programs (Launcher, EADM, etc) are closed-
Double click the Sims 3 icon on your desktop, or click the Sims 3 shortcut in your start menu. This should automatically start the Sims 3 Launcher, as well as the EADM in the background. Don't worry about the EADM, since in the Sims 3, unlike the Sims 2, it just acts as a middleman between the Store and your Launcher. Just leave it minimized in your system tray (where it should appear as a little green circle).
Now that your Launcher is up, open your browser (IE7, IE8, or recently updated Firefox or Mozilla preferred), and go to the Sims 3 Store page. Log in to the account on which your game is registered.
You can download free content from the Store like Riverview and the promotional Prius, or you can purchase new content from the Store, or you can get user-made content from the Exchange. If you've already purchased items on this account, you can re-download them by either going to their page (where the "Buy Now" button should have been replaced by a "Download Again" link), or going to the "Purchase History" page under the "My Account" tab on the bottom left side of the Store page (the site may ask you to re-enter your password when you try to go there).
When you try to download an item for the first time, the site will produce a little pop-up asking if you have the Sims 3 and the EADM installed. Check the "yes" box and confirm. Your browser and any anti-virus software may also produce pop-ups, asking if you want to let the site access a program on your computer, or to confirm whether you want the file being downloaded to be used by the Launcher, etc. Just click "yes" or "confirm" to all of these popups.
Your download should now appear in the Downloads section of the Launcher. Let the download complete, and it should show an entry for it with a thumbnail and the item's name. If a big red circle-and-slash symbol appears there, however, it means that there was a problem with the download. In that case, simply select the download using the checkbox next to its name, hit the "delete" button in the Launcher, and attempt to download the file again. If problems persist, you may be having a Proxy or Firewall issue, or the site may be having problems due to traffic. Wait a while, try again, and if its still broken, try posting in the "Technical Discussions" section of the Store boards.
Once the content is downloaded, you can either hit the "Select All" button in the Downloads section of the Launcher, or manually select each piece of content by clicking the checkbox next to its name (the entry for the item should highlight as yellow while its selected). Once you've selected all the content you wish to install, hit the "Install" button in the Launcher.
In a few moments, the Installer program should start up and appear as a new window. It will take some time to finish starting up, and then it will begin installing all your content one by one. Please be patient while this completes. You cannot install content while the Sims 3 game is running, so make sure that it is shut down when you attempt to install content. Once the install process completes, hit the "OK" button, and the installer will close automatically. You should then be able to click on the "Installed Content" button on the side of the Launcher, and in that section you should see the content you've just installed to your game, as well as the options to uninstall it.
Now, press the Play Now button in the Launcher, and your game will start, and your content will be waiting for you. Build/buy objects will appear in Build/Buy mode, custom presets will appear in the Create a Style tool for an object or outfit, hairs and clothing will appear in CAS, Families and Lots will appear in the Sim Bin, and individual pre-made Sims should appear in CAS in the "Pre-Made Sims" section.
If you want to download content to one computer and then install it on another (for instance, if the computer with your Sims 3 game on it does not have an internet connection), you can only do this with Exchange content (by using the "Save File" link under the "Add to Game" button on the content's page), because Store downloads have to be verified when downloaded, and thus needs the EADM and Launcher to work. Just cut-and-paste the Exchange content into the My Documents\Electronic Arts\The Sims 3\Downloads folder, start the Launcher, and the content should appear in the Downloads section.
I hope that helps, and have fun playing the Sims 3 with your new downloaded content.
This message was edited 3 times. Last update was at 06/24/10 05:02 PM