"PM" on its way to you, shortly.
However, if you have an FAA multi engine rating and your log book shows "remarks" of a minimum of 3½ hours flight training on single engine operations, and at least 2½ hours of multi engine operation, you can then go to any JAA multi-engine approved FTO (Flight Training Organisation) or RF (Registered Facility) and do a conversion.
The conversion is in two parts - a written examination based on the aircraft type - and a flight test with a JAA Examiner.
There was a question raised recently - saying that to do this conversion you must have 100 hours on twins (I'd not heard it before). Check that with the CAA before you spend the money at an FAA school.