Evan Palmer try these steps
1. Download RFD900Modem Tools.exe
2. Plug in your RFD900 modem and read settings make sure only Mavlink is check also select your preferred Net ID if you want to and write settings.
3. Plug in your 3DR radio and read settings do the same as step 2 and write setting.
4. Plug in your 3DR radio to your plane/quad. Plug in your RFD in computer and open Mission Planner.
5. In mission planner go to Initial Setup --> 3DR Radio --> load setting. If both radio setting air and ground appeared means your radio now is ready for connection. If not means you have to flash your RFD900 ground radio with firmware.
One way is to downgrade the firmware to 1.6 then reflash it with 1.9