Now that java.net is fully shut down, the "download separately" link in the documentation is incorrect. It needs to point to a Backstage location that contains the opendmk.jar file.
17.2. SNMP-Based Monitoring
OpenDJ lets you monitor the server over SNMP with support for the Management Information Base described in RFC 2605: Directory Server Monitoring MIB.
OpenDJ SNMP-based monitoring depends on OpenDMK, which you must download separately. Install the Full Binary Bundle by using the GUI installer, which requires that you accept the Binary License for Project OpenDMK. OpenDJ directory server that you download from ForgeRock is built with OpenDMK, but due to licensing OpenDMK is not part of OpenDJ. SNMP is therefore not enabled by default.