So I got a SSD and I installed windows to it, but now I'm trying to install stuff on my D drive, Harddrive, and its grayed out when I try to install stuff on it. Isn't there a way to enable this? I forgot how to do it, but I need some help. Any advice?

Also how do I route downloads to my hard drive so my SSD doesn't get filled up?

You need to also format your harddrive so that windows knows how to use it. As for putting downloads in your D drive that should just be one of the settings in your web browser.