Hi
I have no real experience with OpenDMK, but did a quick test and seems that integration should be like this:
make MIB file
generate java stubs using mibgen
implement these stubs to return real data
copy MIB file to ${prefix}/share/netxms/mibs (c:\NetXMS\var\mibs on Windows)
recompile all NetXMS MIBs:nxmibc -d /usr/share/netxms/mibs -o /usr/share/netxms/mibs/netxms.mib
restart management console, it should download updated netxms.mib from server
add your server to NetXMS
configure DCIs (set source to "SNMP Agent")