Don't have a clue about Synk, but you can try adding the following line to the top of their file (under the JEXEC statement, which hopefully is there):
jimport('joomla.user.helper');
JUserHelper is a core Joomla function, which we use to generate the activation token for users and a few other things.
Hopefully, including the import statement above will let Synk find it, and thinks will be hunky-dory from there. If not, let us know, and good luck!
Alex