Comparing the current outputs from a previous one is NOT yet available both in Device.CLI activity and Report.ARP table activity. I believe this feature is only available in Device.Backup Running Config. I have tried to perform this on my CatTools Server and it works like a charm. Feel free to perform the following:
1. Use Generic.Device as Device Type and put the necessary fields like IP address and passwords.
2. Under Variations -> Additional commands, Enable Show Running Config and put show arp or related command. See screenshot for more details.
3. Save and run your Backup Running Config and see the results.
https://www.dropbox.com/s/j9g76ohslddoqet/varaitions.png?dl=0