Hi jprim,
You would need to get the current selected tab first via,
http://code.google.com/chrome/extensions/tabs.html#method-getSelected
Then you use the tab.id, that the callback has fired, and updating it with a url:
http://code.google.com/chrome/extensions/tabs.html#method-update
For example:
chrome.tabs.getSelected({}, function(tab) {
chrome.tabs.update(tab.id, {url: 'http://google.com'});
});
If you want to let every link in the popup page to update the current tab opened. You can do the following (as you mentioned within the comments but with currentTarget):
$('a').live('click', function(e) {
var href = e.currentTarget.href;
chrome.tabs.getSelected(null,function(tab) {
chrome.tabs.update(tab.id, {url: href});
});
window.close(); // To close the popup.
});