Check the Project Synchronization Setup under the Transport menu. This is where you chose which of your MIDI Port(s) will receive (or be able to transmit to Cubase) various available control messages, MTC, MMC, MIDI Clock, etc.
I am not familiar with the DR-550 in particular, but a drum machine also needs to be set to some form of “slave” mode (so that the control messages from Cubase will control it, rather than its own buttons only).