I've never used a site like yours, pre-built by another company. Your first port of call should be the admin panel to see if you can do what you want it there. You could also just ask the template supplier.
Your problem in hand editing the code it that it's a dynamic site programmed with PHP. Because of that you'll have to wrap a normal HTML hyperlink in the appropriate PHP to 'print' the hyperlink onto the page, but you also have considerations about how to target the destination of the link.
Without seeing the full code of the page and knowing the hyperlink you want to add I'd be stupid to try and advise on this with sample code. I'd say you should fully explore the admin panel and email or call the support department of the site supplier.
There's not much you can do to a plain vanilla HTML site to break it and even if you manage to mess things up it's relatively easy to fix things. It's far harder with PHP or ASP though, and the consequences of things going wrong in your case could bring your site down.
I'd be surprised if your admin panel doesn't have a feature built in to allow you to modify page content and add links.