Blue Prism is not a computer system monitoring tool - you need to figure out how to do what you want using either command line or another application, and then how you can use that method via Blue Prism.
I have just googled 'cpu use using command line' and the result told me that the following command line would give me CPU use: wmic cpu get loadpercentage
If I wanted to do this via Blue Prism I would maybe use the Start Process command, and in the arguments pipe the result to a text file that I would then read?
You could probably do something cleaner using Code Stages. Maybe create your own object called something like 'System Monitoring' with all the actions you require and using .NET code to get the system diagnistics you need.