The sequencer on the JX-3p is clocked by the seq in 1/4in jack on the back of the synth not midi. you need a regular sound like a rimshot or some other trig pulse to make the sequencer advance.
I think that the Dr550mkii only has midi in and does not send midi sync anyway. So perhaps not the Drum machine you need to run the sync of the JX-3p. Its a slave drum machine and with 2 outputs is ok for jamming but perhaps getting something with more outs would be better. A Roland R8 or a cheap MPC is a good investment.
If you are running a computer setup with a soundcard with multiple outputs, you can set up a trigger sound (eg. a rimshot or a sample of a trig sound) on an audio channel of your Sequencer Just set it to 8th notes or 16ths and the sequencer will be in time, just route it to one of the outs connect a lead and k'blamm cheap sync.. (remember to mute it in the main mix.)
+++ Dont be scared honey, thats just the resonance knob +++