** Can anyone point me to where I can find the jBPM designer tool for use within Guvnor v5.3.5? Instead of getting a download option, all we get is the error message telling us that Guvnor couldn't find the designer tool. **

Older versions of the jBPM Designer are available for download from Sourceforge: http://sourceforge.net/projects/jbpm/files/designer/
Looking at the version matrix: http://docs.jboss.org/drools/release/latest/droolsjbpm-introduction-docs/html/compatibility.html you should be good to go with Designer versions 2.0.Final, 2.1.Final, 2.2.Final, or 2.3.Final

HTH
Tihomir