Add Custom Attributes with Weblogic Server 8.1

I tried to add "Custom Attributes" to a Weblogic Server 8.1 monitor, I followed a link present in the Application Manager 6.0.1 web console then I chose a domain (in my case mydomain) and I clicked on "Show MBeans".
The web console answered with the following messages:

"Error ocurred while connecting to the agent at host gandalf and port 7001. Please try again"

"There are no Management Beans (MBeans) for the selected domain. Try adding from another domain or provide another filter criteria."

I tried to go "Back to Details Page" and I noticed in the "Monitor Information" that Application Manager showed Health of my WLS as it was critical:

Root cause:

"Resource is down.
Health is critical as the resource is not available"

But my server was still working well. (I could access to weblogic web console)

Just for info, I succefully had had access to my Mbeans in my WLS instance with wlshell.

PS:
With web console:
-I tried to access JMImplementation ----> OK
-I tried to access Security ---> it didn't seem to work too.
-I tried to access AdventNet_Agent ---> OK (of course)
-I tried to access WeblogicManagement ---> OK
-I tried to use some Filter Criteria ---> it didn't seem to work too.

Thanks in advance.

Stefano