Ah, OK. Yes, that will update the default value, but the file you've got is just a resource file and doesn't do anything active.
If you really want to have a go at hacking up the code and getting the value of the meta description, you could try doing it with a button and some javascript.
To get you started, insert this bit of code into your copy of edittags.ascx, next to the 'txtHeader' declaration.
create a button using the <button> element, and in the onlick event, put a bit of code in that looks for the content of the header using document.getElementById - the id is 'MetaDescription'. THen find the text box by id or name, and set the value of the textbox to the value of the header on the button click event.
I did post the javascript but the editor kept eating it. So you'll have to work it out this way!