I'm curious if you check the Version information
System->(More)->(Service)->(Version), if the power meter thinks it is the E4418A instead of the B. Since you have changed the DSP board more than likely the memory was reset as would happen during the security/memory clearing process. Since the A model did not have a serial port, this would explain why none of the menu structures are present.
If this is the case, it is necessary to reload the information using SCPI commands using GPIB. The process is discussed at this link:
http://www.home.agilent.com/agilent/edi ... SR.General