Someone else asked a similar question here: http://thwack.solarwinds.com/thread/49393
You *can* set up an alert on a single CPU. It would look like this:
Start a new alert. Set it to be a Node alert. Then change it to be a Custom SQL query (with the sub-type still Node).
In the box where you can type your query, add:
join CPUMultiLoad on Nodes.NodeID = CPUMultiLoad.NodeID
where DATEDIFF(mi, CPUMultiLoad.TimeStampUTC, SYSUTCDATETIME()) < 5
and CPUMultiLoad.AvgLoad > 85
This will trigger when any single "CPU" (core or otherwise) is over 85 within the last 5 minutes (ie: one polling cycle)