Hi! We are trying to use a MODX playing sequences and arpeggios (in pattern mode I think), and we want to use midi clock out to send to a Boss DB90 metronome, which will provide a click track for the drummer.

It will be very important to know what you are attempting to synchronize. Sequences are different from Arpeggios

Sequences consist of data already written to the MIDI Tracks of the MODX.
Arpeggios consist of data that you interact with in real-time.

This means syncing the sequencer data with external devices is as simple as making the connection (MIDI Out -> MIDI In), setting your metronome to slave to the MODX (as master clock).
Press [UTILITY] > “Settings” > “MIDI I/O”
Set the MODX MIDI I/O = MIDI and MIDI SYNC = Internal
Starting the sequencer by pressing the Play button, will start the clock

It does not start the Arpeggiator. The Arpeggiator requires, additionally, a musical input in order to start. That is in the form of Note-on events, generated by pressing keys on the keyboard at a musically appropriate time.

Sync’ing the Arpeggio additionally requires you triggering the controlling event on the beat. If you are late playing the keys, the Arp Phrase will be behind the beat. Unlike the Sequencer, where start and stop is automatically coordinated, the human factor comes into play when you are introducing arpeggios.

When the clocks are sync’d they will run referencing the same tempo... it’s like two interlocking gears. If you link them at the right moment the sprockets stay linked, but if you link them out of sync, they remain locked but slightly offset.

If you are having problems with the sequencer that’s one thing.
If the issue is with Arpeggiators that is a completely different thing.