How do I install Windows with Bootcamp? I am using El Capitan on a 15" MacBook Pro mid 2015. I downloaded a Windows 10 iso file from the Microsoft website. 

Then I used Boot Camp Assistant and selected the file. I allocated 120 GB to the Windows partition. It did it's thing and then restarted automatically. It booted back to OSX.

When I look at About This Mac, I see the Bootcamp partition.

If I hold the option button while the Macbook reboots, it only shows me my Mac OS partition.

Did something go wrong? It's like Windows 10 didn't install.. what should I do from here?
Boot Camp Assistant is a glorified partitioner. It doesn't do any actual installation of anything. For that, you'll have to actually launch the Windows installer from the ISO, by burning it to a disk first, and booting off of that.