Too bad, nobody answered - by now I have the same question!
We use the Business Rules object in ARIS and connect it to the applicable function on the EPC diagram.
However, we do not have experience with the Business Rules Designer tool.
Thanks für your information; so far we do the same, but we use the object type entity type, since we have to document additional information with it - actually we place a use case connected with the business rule (as used in the respective application) in the EPC, so that as well business as IT knows in which function in which context which Business Rule (BR) applies.
But in another case we may need to define the BRs first and connect them to the existing target process. This is far more difficult to accomplish.