Grid and arc have no midi interface. They communicate with computer with OSC protocol.
To let your monome device drive your preferred virtual synth you need a monome application from monome site or you can create your own application, for example with max or collider.
If you want to use monome device with your modular synthesizer, best use is one of monome dedicated modules, most versatile is Ansible.
The old way to use monome with a modular synth is connecting monome device to a computer running an application to convert OSC messages in midi messages to your midi interface (parc, poligomè, press cafè, ecc), then to a midi module like Mutable Yarns, for example.