if you are looking for a small number, you can type their name right in the line where the filter is. A larger amount you may want to figure out a filter or list...

(Actions-> Agents/Plug-ins->Push Altiris Agent)

Then I believe you also need to Discover HP computers on machines that have the agent, and then you send the HP Client Manager Plugin to them.

Settings->All Settings->HP Client Manager->HP Client Manager Configuration->Discover HP Computers - First, and then you can go to
Settings->Agents/Plug-ins->HP Client Manager Agent->HP Client Manager Agent Install

Once the computer was "Discovered" as an HP computer, it will add to the count of "Resource Target of Discovered computers with no HP Client Manager Agent."

It's a kind of long process, but once you understand the thought behind it things get easier.

sure... once the Altiris agent is on, you need to run the discovery task, and once the PC has been recognized as an HP, you can push the HP client.

Have you looked in Settings->Agents/Plugins->HP Client Manager Agent->HP Client Manager Agent Install and turned that on? I believe it is turned off by default.