This error could be caused by a few different problems.
First, make sure that the DB2 environment variables are set in the process that starts and runs the agent. Make sure the .bashrc and/or the .profile contain the command to source the db2profile script. One of the most important environment variables seems to be LD_LIBRARY_PATH, which is set by the db2profile script. The export command with no parameters will show if the DB2 variables are set. If they are all there, you should be able to start the agent from that process.
Make sure the DB2 instance is running when attempting to save/activate the Hyperic configurations.
When defining the Hyperic service for DB2, don't confuse the DB2 install path (which is usually under /opt or /usr/opt) with the instance path (which is the instance owner's home directory).
The LIST NODE DIRECTORY command does not show the local instance by default, so you may want to CATALOG LOCAL NODE to help the agent see something in the node directory.
Although it's easier to run the agent as the DB2 instance owner, that approach may not sit well with some administration policies, so you should be able to use a different account to run the agent and connect and/or attach to DB2 as long as that user has at least SYSMON and has properly sourced the db2profile script.
