I recommend using the Assembly Browuser Window (JustCode Menu - Windows - Show Assembly Browser Window). If you have a reference to that assembly it will be in that list. If you don't have one, you can add an assembly from the second command in the toolbox. Then right click on the assembly and choose Decompile. This will decompile the assembly and will generate a new pdb file matching the newly generated source. Then just wait for the JustCode analysis to get refreshed and you will be able to navigate to the decompiled source (Go to navigation, Go To Type, etc) and put a breakpoint.
If you need further assistance, please don't hesitate to write back!
Check out the Telerik Platform - the only platform that combines a rich set of UI tools with powerful cloud services to develop web, hybrid and native mobile apps.