For now just consider,diagnostic agent are your SMD agent.
In fact, most of the system might have diagnostic agents installed and solman too. But you have to connect them to solution manager system, and that is done using smdsetup script.
for that:
goto usr/sap/<SID_AGT>/SMDAXX/script
smdsetup managingconf hostname:"sapms://solman.full.qual.host.name" port:"81XX" user:"SMD_ADMIN" pwd:"XXXXXXXX"
If agent are not installed than install them.
In every system they should be installed under user DAAADM and path /usr/sap/DAA.
Regards,
Divyanshu