You've already downloaded jQuery above the pop-up, so there's no need to download that again. You are missing the actual code that flips the
display property of the overlay/pop-up, and there is a pretty good chance that the pop-up box HTML is supposed to be living inside of the overlay div.
A BIG note here: the code for the document type should be:
Note that there is a space, not a period. That one line of code tells the browser which of the various HTML dialects the rest of the page is using (there have been many over the years, all subtly different), and what rules it should use to interpret and display your HTML and CSS. As was (almost) pointed out above, the browser is interpreting your existing doctype declaration as a "foreign" element (an undeclared SGML element entity), so it's being ignored. That probably means your page will be interpreted as HTML 4.01 Transitional, where you want it interpreted as HTML 5.