Problems monitoring servers with OpManager

I've enabled the SNMP on all servers but aside from one server, I cannot view the graph or whatever information regarding the servers. I'm currently using OpManager to monitor 4 windows 2000 server and two Solaris 8 servers. Although all the servers have SNMP enabled, OpManager doesnt show the blue star indicating it. One of the servers with a blue star indicating SNMP enabled doesn't even show the graphs for the Resource Monitors and I cannot generate the report for the server. Can anyone tell me why?

Another question is how to configure the threshold value for the monitors? How can I know what value the threshold value and the rearm value refer to (in term of unit)?

Any help is very much appreciated as I'm using OpManager for my school project and I have a few days to fix this problem.