I’ve just checked the plugin and found that it bundles Meta Box inside:
However, they don’t keep the original version of Meta Box. Instead, they changed the code (prefix in the screenshot above). That might break the compatibility with Meta Box extensions.
If you’re using their Pro version, please contact them to either update their code (the modified version of Meta Box) or include the original version of Meta Box.
PS: I’ve posted a topic on wordpress.org.