You're in for the night - is it tonight, May 27? If so, forget about eating anywhere. It's Brown graduation weekend and you can't get a table anywhere. If it's some other night, then I'd suggest Hemenway's, which you can probably walk to. No other obvious choices if you have to have lobster.
Note that CH is not a concierge service - if you pop in in mid-afternoon to ask about plans for that evening, you will most likely be disappointed.