my horizontal menu isn't showing up horizontal :( …help please

I am using theme1621 that has a horizontal menu as seen in it’s sample page here: https://www.templatemonster.com/wordpress-themes/39694.html

When I uploaded the theme and started working on it via WP, the header nav is only showing up vertical, instead of the horizontal it shows in the example page. See it here: https://www.liamack.com/

am I supposed to go into the CSS and do something to make it go horizontal? did I screw something up in creating the menu via the control panel? or is the theme broken and I have to fix it, and how?

Thank you very much for any help! I appreciate it ??