Do you mean E2 to this MK2? If so do this:
Go to global settings on the E2 and make sure that the clock is set to internal
Connect a MIDI cable into the MIDI In port of the ER-1, to the MIDI Out of an external MIDI device.
Press the MK2 [GLOBAL] button.
Press the Down or Up [CURSOR] button until the Parameter select LED is on the same line as “Clock“.
Rotate the Data Dial to select “Ext“. This will sync the ER-1’s MIDI Clock to that of the external MIDI device.
Press the [WRITE] button twice to save these settings.