You need to add the SSL certificate (public key) to the "cacerts" file of the JRE that is used by Workbench. Here is a sample command line that should work, adjust the path info to match your environment.
keytool -import -file "path to the public key .cer file including .cer file name" -keystore "C:\Program Files (x86)\Adobe\Adobe LiveCycle Workbench ES2\workbench\jre\lib\security\cacerts" -storepass changeit
Thanks Steve I consulted the preparing your dev environment PDF which stated a similair command line prompt.
The instruction said a restart of Workbench was sufficient. That wasn't the case, but after a reboot of the machine the certificate was accepted an trusted,
You really should not have to reboot. When I added a cert to the cacerts file, I only had to restart Workbench for the change to be recognized.
I'm glad you got it working.