You need to be clear about what you want. First you said that the breakpoint is not working in the function module that you are using the method, but now suddenly you want the debug rule function, which is completely different and separate thing. As Vijay said, just open the rule in PFAC, and simulate it there.
If you are not able to reproduce the problem though simulating the rule, the final thing is to use the well known trick for debugging a background process. This recent blog gives nice step by step instructions:
http://scn.sap.com/community/bpm/business-workflow/blog/2015/01/12/workflow-debugging-trick
Regards,
Karri