Sentinel uses the RR Database, so start with this search result here:
Massachusetts Scanner Frequencies and Radio Frequency Reference
Those links are raw data dumps from the FCC license data, and won't be in the RR database (and therefore not in Sentinel as the OP is looking for) unless actually verified and submitted to the RR database admins for inclusion in the actual RR database.
To the OP... You will find a few National Grid frequencies in the actual RR database at this link:
National Grid Trunking System Profile
www.radioreference.com
And those are in Sentinel, look in:
USA --> Massachusetts --> Essex --> County Systems --> National Grid
From a while ago I seem to remember that National Grid in that area has now mostly moved to an LTR Passport System (not 100% certain of that). Again, local knowledge in the Massachusetts forum would be your best bet for current, accurate info.
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