So I was following these instructions on the official site.
Download this hardware configuration archive: breadboard-1-6-x.zip, Breadboard1-5-x.zip or Breadboard1-0-x.zip depending on which IDE you use.
Create a "hardware" sub-folder in your Arduino sketchbook folder (whose location you can find in the Arduino preferences dialog). If you've previously installed support for additional hardware configuration, you may already have a "hardware" folder in your sketchbook.
Move the breadboard folder from the zip archive to the "hardware" folder of your Arduino sketchbook.
Restart the Arduino software.
You should see "ATmega328 on a breadboard (8 MHz internal clock)" in the Tools > Board menu.
Once you've done this, you can burn the bootloader and upload programs onto your ATmega328 as described above. Be sure to select "ATmega328 on a breadboard (8 MHz internal clock)" when burning the bootloader. (If you select the wrong item and configure the microcontroller to use an external clock, it won't work unless you connect one.)
I followed all of this including installing all of the three zip files, rebooting it multiple times, but the "ATmega328 on a breadboard (8 MHz internal clock)" won't just show up in the Boards menu.
Any help would be appreciated!