Hello Jam,
 
That is an error related to PHP version used in your hosting server, because that function is only available since PHP 5.3, so you are probably running WordPress with an older PHP version in your servers (5.2 as it is the requirement for WordPress).
 
We will upload a quick fix to allow users with PHP 5.2 to use this plugin in the next 24 hours (we will also send you an email with this new plugin version)
 
Anyway, if you can manage to use a newer version for PHP in your hosting, we really encourage you to do it so. We know there is still some debate about this issue inside WordPress community, but we actually support that using a version that is not supported anymore since almost 4 years ago (http://php.net/eol.php) is not doing any good, neither for WordPress itself and the community of developers (security reasons, new functions and approaches to coding in PHP, etc), neither for end users that could find easily and more and more often this kind of errors with a lot of modern plugins and themes that are being released nowadays.
 
In the case that you manage to upgrade PHP version (or if you ask to your hosting provider to do so for you) you could enjoy this plugin even this very day.
 
Regards and thank you for trying pro version of this plugin.