I\'ve gone through everything on http://www.chromium.org/chromium-os/android-apps and also watched the IO video and there doesn\'t seem to be any mention of what the side-loadin
As of November 2020 you can now run ADB on your chromebook using just a web browser.
Check out https://webadb.com to learn more
Edit: The instructions below are now no longer needed as ChromeOS has enabled allowing ADB access to the Android container without needing to put the Chromebook into developer mode BUT ONLY IF you want to access ADB from the Linux container on the same Chromebook as documented here.
Unless you are in Developer Mode, you won't be able to enable unknown sources. So in order to side load apps you will need to put your device into Developer Mode (instructions here - follow the steps for the Chromebook Pixel 2015).
Once you are in Dev Mode, go to Chrome Settings > App Settings > Security > Unknown sources (move to the right)
After enabling developer mode you can side load apps in one of 2 ways:
Here is a sample walkthru of setting up an ethernet connection for approach 3: