Once you completed all the steps to create a new button and add to the Page Layout, still cannot see the Dynamo button in Lightning Experience. Check whether you can see it in Classic UI. If the button is visible is Classic UI, the chance is that you have activated the option to override the custom setting in Page Layout for Lightning Experience.
In the Page Layout, Salesforce1 and Lightning Experience Actions area, check whether you see there are Buttons in the section.
If you activated it by accident, you can revert the Defaults. The button should appear again on the page after that.
If you want to keep the customizing option, drag the button from Salesforce1 & Lightning Actions group into the Salesforce1 and Lightning Experience Actions area.