Hello,
If you lookup your system in SIM go to the second tab tools and links. Here lookup your credentials, i would think that you would only see a trust and SNMP credential set.
If that is the case, go to the option menu and find the menu option credentials and set your WBEM credentials under the credentials bit. If you filled that in reidentify your server. If you take a second look at the server credentials you should see the credentials you just filled in at the general settings. Now every new system will try these credentials for wbem and SSH if you got both protocols turned on.
The whole problem is that you have to set the WBEM credentials in SIM to, not only in the WMI Pegasus tool.