You won't get a 'postcodes' screen up - if you are referring to one of the recent posts about 7-digit postcodes, it was a fudge based on someone compiling UK postcodes as POI's and creating a copy MAP DVD with those included.

To enter a 5-digit postcode, you do that from the Destination Input screen, and your nav should ask CITY/PC - you just start entering the postcode letters/numbers in there.

Oh, and welcome to the forum BTW