Text that doesn't appear on the contribution page
The text that you add to the "Pay Later Instructions' box should show per the following:


  Instructions added to Confirmation and Thank-you pages, as well as the confirmation email, when the user selects the 'pay later' option (e.g. 'Mail your check to ... within 3 business days.').


You said:


  what I wrote in the editor doesn't appear on the "contribution" page


Unsure if you incorrectly typed 'contribution' instead of 'confirmation', or if you meant what you typed, in which case the text is not expected to show.

The other text "Pay later label" should show on the Contribution page, but may only after an amount has been selected if (assuming more than one payment option is set up)

Please add any clarification you can to your original question via 'edit'