Migrating Mail.app rules from 10.6 to 10.8
I copied all of the user-created &lt;dict&gt;...&lt;/dict&gt; items from my old MessageRules.plist and added them to:

~/Library/Mail/V2/MailData/UnsyncedRules.plist


When I launched Mail.app again, they were in my rules, but disabled. One click on each and they ran fine.

Note: Simply replacing the entire set of &lt;dict&gt;...&lt;/dict&gt; items in UnsyncedRules.plist will not work. You must add them into the current set of items in order for it to work properly.