I have trouble with using capybara to test tinymce form. I\'m using tinymce-rails and have 7 editors in my form. Also I\'m using asciimath plugin with tinymce.
Try to switch to an iframe that contains tinymce textarea input, and than send_keys:
# +session+ is an instance of Capybara::Session class
browser = session.driver.browser
browser.switch_to.frame(iframe_id)
editor.native.send_keys(text)
browser.switch_to.default_content