Create a new Activity and select Device.InterDevice.Ping as the Activity Type:
Leave the other options on this tab at their defaults.
Select, for instance, "Every minute" as the Recurring option in the Times box to run the activity every minute.
Leave all other options on this tab at their defaults.
Select the Devices you want to perform ping testing from.
Set preferences for reporting on this Activity.
Either, enter a list of Host names and IP Addresses in the Ping a list of hosts (Display name, IP address) text box or simply check the Or, ping all other selected devices option (checking this option will disable the Host names, IP Addresses field the two are mutually exclusive).
Only record ping failures: tick box, will only log ping failures.