Newbie here. Unlike the default button in library->input, I can't double click on the button in Assets->Android4 (or anything else with text in it) and get the inplace text editor to appear. What am I doing wrong?

Android buttons are components so the workflow is a little different. A component can be customized by editing its instance and overriding individual properties. To override properties "enter" the component using double-click. Hit Escape to exit component editing.

In case of Android button you'll to double-click once to "enter" the button, then double-click once more to edit the text. Let me know if this works for you.