At present, the plugin only uses the excerpt for the page description … it adds the excerpt box to Pages (normally it is only available on Posts) to allow a custom excerpt to be created, so the easiest option would be to fill this field with the same contents as you are specifying for the meta description. (You may need to select to show the excerpt box from the Screen Options tab at the top of the dashboard.)
What plugin are you using to populate the meta description?
Since there is no standard field for the meta description in WordPress, I guess this could probably be achieved if I created an option to name a custom field to use as the description – as most plugins would use a custom field to store the information. If the field didn’t exist (or was empty), the fallback would then be to use the excerpt or page content as normal.
I’ll consider this option for future releases.