Hey Liz,
You need PHP 5.4 or above which should really be the minimum standard these days due to security issues.
PHP is the programming language on your server and is nothing to do with WordPress. You will need to speak to your webhost about getting your PHP version updated.
Having said that, the PHP requirement has been in place for almost 2 years now on my plugin so are you sure this is your issue? Is the plugin active in your admin and can you get to the admin area? If so, then the PHP version is not the issue.
Have you been through the below FAQ to see if any of these are the cause?
https://responsive.menu/faq/why-wont-my-menu-show/
Peter