By default the logs are in profile "logs" directory
I your case
/opt/IBM/Websphere/Appserver/profiles/DMGR/logs/dmgr
If you have more profiles then
/opt/IBM/Websphere/Appserver/profiles/PROFILE_NAME/logs/
nodeagent
server1
SERVERNAME
You have all the logs (SystemOut.log, SystemErr.log, native_stderr.log, native_stdout.log, startServer.log, stopServer.log,...) inside each directory
Hope this helps. Tell us if you need more support
Regards