I added instructions on how to download different iOS versions. I’ve since updated the instructions to reflect this. Xcode 7 has been out for awhile now, and these instructions are still valid but a new user may not realize that. Xcode is a free download from the Mac App store.
You’ll need to download and install Xcode for OS X to access the iOS simulator.
I’m revisiting one of my more popular posts, Using Xcode iPhone Simulator to Develop Mobile websites, which is nice but a little out of date.