Add a new health test or Edit the health test that you want to add this new counter.
Click your version of Exchaqnge on the left and scroll down to the performance counters section.
Click the "Add Performance Counter" link.
Specify "\\servername" for a server that has this counter. --This is used to retreive a list of counters.
Then click the dropdown under Performance Object. This will list all the permformance objects on the above server.
Then find performance object/counter/instance you wish to monitor.
Click OK to add.