From the spring '16 release notes
Custom buttons and links that define their content source as URL are now supported in Lightning Experience and the Salesforce1 mobile app. This feature was previously available in Salesforce Classic only.
Seems straight forward. Create the button, add it to the classic page layout, see it in lightning. But for the life of me, I can't figure out how to get this thing to show up.
What am I doing wrong? Seems like I'm following the instructions outlined in this trailhead. Even though it's in the release notes, it's not mentioned in the help docs on actions in lightning. Other peopler have had issues and have gotten it working eventually. What am I missing? Are there other factors to consider? Are those documented anywhere?