@Nena16lovex Don't worry about the delay. People don't always have time to troubleshoot, but they come back when they can, and there's always someone here.
Open a File Explorer window, click This PC in the left panel, and you'll see your two drives listed. I think they might actually be the same drive, but partitioned (as in, someone used software to split a single physical drive into two separate designations). Unfortunately, you can't merge the two partitions unless you either delete the D drive (losing all your data) or you use third-party software. But you can still free up space on C by moving files manually.
Without knowing what you have on each drive, I can't tell you what your options are. But you can look for yourself: right-click on any folder, select Properties, and see how much storage (in GB) it takes up. Your goal should be to free up about 20 GB on C, so look around a bit and see what you can move to D. When you click This PC and then the C drive, you'll see all the folders on C minus a few that Windows hides by default, so you'll be able to quickly find out what's taking up a lot of storage.
The Sims 3 folder in Documents\Electronic Arts holds all your user data, and it needs to be specifically in Documents\Electronic Arts, or the game won't read it. You can move some files elsewhere though, for example if you have a lot of screenshots. They'll open like normal image files no matter where you store them. You can also temporarily move saves you're not playing to a different location, and move them back when you want to use them. There's no need to delete the saves outright.
If you need help figuring out what to move, let me know, and give me an idea of the sizes of the large folders on C.
As for the mod, NRaas MasterController has, among its many functions, the option to focus on any sim currently in the world. So if there is a unicorn somewhere, MC will find it and show you where it is. When you have it installed, click on City Hall and select NRaas > MasterController > Focus, then choose a filter to narrow down the list. "Species" should work, or maybe "Type of pet pool." If you have trouble with this, let me know, and I'll experiment a bit myself, but I've never specifically searched for a unicorn before, so I don't know exactly what categories to choose.
If you've never installed mods before, this is a good guide:
http://modthesims.info/wiki.php?title=Game_Help:Installing_Sims_3_Package_Files/Setup_and_Files