Hi Nelson,
have found you created couple of threads for managed system configuration too,
There are lots of step by step document available for managed system configuration and diagnostic agent assignment
Please review under
http://service.sap.com/rkt-solman ->SAP Components ->Solution Manager7.1
http://wiki.scn.sap.com/wiki/display/SMSETUP/Diagnostics+Agents
regarding your error for
'EHP3 FOR SAP ERP 6.0: Central Applications'
You might be wrongly selected the product instance with the diagnostic agent relevant tag, please run the verification of product instance
1. goto Product system editor
2. click verification from the left panel
3. execute verification check, this will remove the inconsistency of product instance assignment.
Other way round,
1. remove the installed tag from product instance "central applications" from the technical system editor -> software -> product_intance tab.
2. Click the add button search for "select by" -> proposal for un assigned software components now assign the relevant product instances
3. mark the relevant instance with the diagnostics agent relevant tag;
this is the old document, but might be useful for your basic understanding #sapadmin:: How to assign Product System in SOLMAN 7.1 & How LMDB, SLD, SMSY and Landscape Verification work in SOLMAN7.1
Best regards
Jansi