Adding a facebook 'F' logo link [SOLVED]

If you just want to add a link to your Showkase menu (without any further custom code such as an <img> tag), then this can be done within the Showkase interface by creating a new 'Navigation Link' page.
The new navigation link will appear in the menu at the top of all your Showkase pages but there will be no actual Showkase page associated with this link within your site.

If you want to manually add a link to your Showkase menu (which you would need to do if you want to add an image to the link), then you would need to edit each and every one of your Showkase pages. Showkase creates static and complete HTML pages (it does not produce a separate and individual menu file which is included via server-side scripting into all your pages). Also, please bear in mind that any manual modifications will be lost if you update the pages within the Showkase interface.

Each Showkase page has its own folder in the root Showkase directory.
For example, if your Showkase directory is named 'showkase' and you create a page named 'Page Number One', then the page on your web server that you would need to edit would be 'showkase/page-number-one/index.html'.
If you open the page in a plain text editor, search for your existing menu entries and you should find where to add a new one with your own custom code.

If you want Showkase to add a custom link at the end of your menu on every page each time the 'Publish' button is clicked, you would need to to open the 'showkase/admin/classes/nav.php' file in a plain text editor, scroll down to line 177 and just before the return $navDom; line, construct the HTML you require within a <li> tag using PHP DOM techniques and append your custom DOM element(s) to the $navRoot variable.
For example, if you wanted to insert a link which displays the text 'Click Here' and directs the user's browser to the URL http://www.example.com/index.html, then you could use the following code:

$customLi = $navDom->createElement('li');
$customA = $navDom->createElement('a');
$customA->setAttribute('href', 'http://www.example.com/index.html');
$customText = $navDom->createTextNode('Click Here');
$customA->appendChild($customText);            
$customLi->appendChild($customA);
$navRoot->appendChild($customLi);

Please note that the source code of Showkase was not intended to be modified by users in such a manner and I do not know what (if any) knock-on effects this may have.
More information on using the PHP DomDocument class can be found here.
I hope this helps.

If you want to use an image link (instead of a text link), try something like the following. Make sure the path to your Facebook logo image is correct.

$customLi = $navDom->createElement('li');
$customA = $navDom->createElement('a');
$customA->setAttribute('href', 'http://www.example.com/index.html');
$customImg = $navDom->createElement('img');
$customImg->setAttribute('src', '/facebook.png');
$customImg->setAttribute('width', '50');
$customImg->setAttribute('height', '50');
$customImg->setAttribute('alt', 'logo');
$customA->appendChild($customImg);
$customLi->appendChild($customA);
$navRoot->appendChild($customLi);