The cross credit is a pain in the backside.
The credit can be given as above when you have 3IFR departures an arrivals on the class/type. The extension can only be given for the remainder of the validity period of the MPA IR from which credit is given. Generally it's not worth a great deal as you are still required to revalidate the MEP class rating in the aircraft and fly an assymetric approach and go around and assymetric approach to land in the LPC with sole reference to instruments which is almost as long as an IR LPC anyway.
You will need evidence of the MPA prof check including signature of the log book entry, the 3 IFR departures and arrivals and your licence. He will then fly the MEP class rating LPC with you (needs to be a CRE/IRR) and sign all te pages. In order to cross credit all IR's must be current, otherwise you must do a renewal LPC in the aircraft.
Cheers