You can only have 5 digit postcode search with unmodified map discs. Enter it into the CITY field. I believe there is a 7 digit postcode hacked map disc. I don't have it.

Upgrading the firmware will not give you 7 digit postcode search.

You can find the firmware here. Make sure you burn it with IMGburn in MODE2/FORM1 at a slow speed. Failure to use MODE2/FORM1 will instantly brick your nav drive. You have been warned.