I'm trying to install the game but I noticed that when you're about to install it, it mentions that the "installer files" are missing. Does anyone know how to fix this?

Did you download all the installer parts and have them in the same directory? There should be 1 exe file and 2 .bin files, total size of about 6GB.

How do I download the installer parts and have them in the same directory?

Exe file, first bin file, second bin file. Click on each of the links, let them all finish downloading (they should be downloaded to your Downloads folder), then run the exe. If for any reason any of those fail to download, delete the partial then download it again.

Actually, it's alright. I was able to run the game a couple days ago.