If this is the Letter of Intent required for the original petition I-129F, then it needs to be addressed to the USCIS where you intend to mail the petition.
If you have already submitted the I-129F and are speaking about an updated Letter of Intent needed for an embassy interview, then it should be addressed to the Embassy/Consulate wherever the Visa interview will be located.
I recommend looking at the guides on this site for K-1.
