Have you gotten a result for that from any query? I’m wondering whether it’s maybe selecting one result from a computer posting multiple results for MAC address.
I don’t have a console handy as I’m offsite this week, but I think you might need something along the lines of
whose (exists "Client Mac Address" whose (it as string contains "xx-xx-xx") of it)
or maybe even
ids of bes computers whose (exists values whose (it as string as lowercase contains "xx-xx-xx" as lowercase) of results of properties whose (name of it = "Client Mac Address") of it)