To add new landmarks to your game, you must first install the latest version of the SC3K Plug-in Tool (in the \ADDONS\SIMCITY\PLUGIN directory). (Note: You only need to download and install the Plug-in Tool once. After that, you can add as many Landmark Packs as you like.)
This is a self-extracting file that will install itself in the correct directory. Run the tool by downloading the file to your computer and double-clicking the icon. Follow the on-screen instructions and when you're done, you're ready to add Landmark Packs to your game!
These also are self-extracting files -- install the Landmark Packs by double-clicking the icon. The next time you start SimCity 3000, the Landmarks will be available in your game.