Ensure your game is patched to (or the latest digital release). The "Command & Conquer" collection and GOG versions are pre-patched.
You can find all of these mods—completely updated and free—on the official community exchanges: 1. Simtropolis Exchange (STEX) stex collection vol4sim city 4 mods disk 4 download upd
Find a mod or building you like in the browser and click its download link (this accesses the file locally from your extracted folder). Ensure your game is patched to (or the