tokenize is a function comes with XPath 2.0. To enable XPath 2.0 functions uncomment the following entry in the synapse.properties file which is located at $ESB_HOME/repository/conf directory
synapse.xpath.dom.failover.enabled=true
then you have to specify the mediator as follows,
<property name="Message" value="a,b,c,d,e" scope="default"/>
<property xmlns:fn="http://www.w3.org/2005/xpath-functions" name="Tokenize" expression="fn:tokenize(syn:get-property('Message'),',')" scope="default" type="STRING"/>