The editor is required to restart for the Editor Toolbar Button plugin to appear. The plugin is enabled and the project is compiled, but the button does not appear.
After further discussions, we are marking this as a Regression from 4.15.2
Result: The editor marks plugin as enabled, but the button doesn't appear after hot reload
Expected Result: The new Editor Toolbar Button appears after the project compiles and hot reloads
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-44161 in the post.