Use the following code to render your TinyMCE Editor:

If you want to populate the Editor with some value from database, retrieve such value first and then assign it to a custom variable and use it like so:

In the above code, $custom_variable will hold the content retrieved from database.

Try the above and let me know how that goes.

Thanks for the code but, I created an text area element with the add_settings_field and I am able to render, type and save text in it. How could I add the editor instead of the textarea?

Thanks in advance.

Good question. Simply add it in functions.php file of your theme, save it and that should do it. I have now updated the post with this info for future reference. Thanks for your input.