Wordpress shortcodes do not display 'insert' button
Probably a typical WP plugin/theme conflict. Try to disable all your plugins except CiviRM and see if you can see the button.  If you can see it, then turn them on in small groups, once you find a group, then do those one-by-one to narrow down which one is causing the problem. If you still cannot see it, turn all your plugins back on and change your theme to one of the WP twenty-themes and see if you can see it.

If it is the theme, you can make your shortcode edits and then switch back to the theme you normally use.