I downloaded a bunch of CS packs, all of the packs do appear when in Sandbox mode but not when I play Custom Scenario. Some are there (in Custom Scenario mode), but not all of them; the weirdest thing is that if I create a custom building with custom CS parts in Sandbox mode or with the Building Designer tool, and then open it in Scenario Mode, those parts will appear in the menu! Why is that? Is not a BIG problem because in Building Designer I could just put ALL the parts of ALL the CS packs I downloaded in the ground, save it as a structure, then open it in my park, delete them and they would become available in the menu; but, it's time consuming, so if you know how to solve this problem that would be great.

It's no error or problem, you actually just discovered a great glitch! The reason for this is because over time, you spend a certain amount of money on research (for rides, scenery, etc). You can adjust how much you spend, but the less you spend the less the research is happening. Kind of like a fundraiser for breast cancer, but for RCT3. The more you research a specific thing, the more it unlocks. Research happens automatically, and can be a slow process. Most CS sets aren't unlocked yet, so you have to wait. You saving it as a structure and placing/deleting it is a better way to do it. You should actually tell Justin to but that under the cheats section!

Oh great! I just found a useful cheat LOL.