Can you clarify your question a bit? There is no way to ...... add agents using client.
If you're wondering why your agents doesn't appear in 'agents overview' there (I assume this is what you are asking), then to begin with, here are the few basic checking you can do:
1. Agent can talk to collector (port 9998, check firewall)
2. Collector can talk to server (port 6698 or 6699, depending on your AppMon version. Again, check firewall)
3. Is the agent up and running? (the way to verify if agent service is running or not depends on what type of agent you use.)