Is there a keyboard shortcut or menu option to edit a selected text box in Keynote?
If you open Keynote and click on the Help menu item there is a line that reads Keyboard Shortcuts. Click on this and the following panel will appear.



in this panel click on the link in the middle of the panel and get this new pane shown below.



Now if you click on Select Text you get the following selection of shortcuts.



Maybe the shortcuts you want are under something besides "Select Text" in which case you can try another selection.