The Challenge: You have to teach a policy that no one thinks is useful
Possible Approaches: Begin by splitting the group in half and having each group brainstorm their responses to one of two focus questions. One focus question should ask them to identify the potential benefits of the new policy. Most people will rise to the challenge and convince themselves. Avoid telling them. Instead, let them come to the same conclusions that led to the establishment of the policy and thereby own the answers.
The second focus question should ask them to identify the problems with the current and/or proposed policy. Dignify their concerns by listing the reasons for their objections. Then assist them in proposing constructive recommendations to address or resolve those objections. Their concerns may be very valid and their recommendations may be able to improve the policy and its impact.
Clearly, there are probably good reasons why the policy is in place. Rather than stating them (because no one likes to be told what to think) it can be helpful to pose realistic scenarios or assign job-related case studies in which the policy has a significant and compelling impact on the outcome. Launch the participants on a “voyage of discovery” and let them determine the value of the policy in “simulated” action.
If all else fails and the group turns on you as the unlucky messenger of bad tidings, you may simply need to say that you are there to explain the policy so that they are aware of it and can apply it effectively- but that you are not there to defend it.