Here are some setting on the DJ that you may need to check.
On Land Manager console, press the SYSTEM key. Select Configuration and press ENTER. Select Serial Port and press ENTER. Verify that Active Config is set to GPS. If it is not select GPS and ENTER. Verify that the GPS selection is 9600-N-8 and Connect=Direct. If you have a PCS, the steps to select the active configuration may not be entirely accurate. However, make sure that the Baud Rate, Parity and Data Bits are set to the values above.
Using the Null modem cable would be correct.
In Mobile, you did setup the Dickey John as a Controller and not a Sensor, correct? You can check by going to Config-Logging-Device Setup. Highlight the Dickey John/Land Manager and see where the dot is placed.
Start a new VRA Job from the Jobs tab to re-load the VRA rate map and see what happens.