Pages: keyboard shortcut for inserting an equation
You can use the keyboard shortcut Command + Enter to invoke the Insert button and press the Esc button to invoke the Cancel button.

If you have keyboard navigation enabled (System Preferences app → Keyboard → Shortcuts → Use keyboard navigation to move focus between controls, refer to the screenshot below), it is generally possible to move focus to onscreen control using the Tab key and invoke the control currently in focus using either Enter or Spacebar key. However, it doesn't appear to be the working in the given case. So the aforementioned keyboard shortcuts should be your recourse.