Many ask this question you must have downloaded both and I repeat BOTH versions for apple and Windows. Afterwards it is in your files download a website on your chrome book like Windows, Firefox, Safari anything that's not the Chromebook Google search engine. 

How do do it: 

go into settings go to "search" 

put in "browser themes"

click home

click search and put in, "firefox", "steam", and/or "safari" or put them all if you want.

This is what it should look like for Firefox 

by the way this isn't the firefox to use it will give you a bug thats why I put it!!! 

Afterwards you had it downloaded go to Windows doki doki and click on it and it will take you to one of the engines you downloaded

and tada!!!