Question: I’m trying to monitor activity of another computer from my computer. I was able to include my computer only. Is it possible in the evaluation copy?

Answer: You have installed both Activity Monitor and Agent on your computer. This is why you can monitor only your own computer.
Free evaluation version of Activity Monitor allows monitoring of only one computer at a time. Uninstall Agent from your computer and delete it from the list. Then you can install Agent on remote computer. It should appear in the list then.

This way you can try the Agent on more than computer one by one. Full version of Activity Monitor allows monitoring multiple computers up to the purchased license size.