Phone the day before to remind them you're coming. You'll probably get someone who says "I know nothing about it" - if so, phone again on the day, then you might get someone on the same shift. (As KT has also suggested.)
As for payment, perhaps being free means they that, paradoxically, they don't actually value you so much. My friends and I do a number of these gigs, and don't ask for a penny for ourselves, but do request a modest donation to our parent musical organisation which has struggled for funding. Or you could nominate a favourite charity. Many of the residential and nursing homes do have a small entertainment budget.

On the whole most places do know we are coming, and are very welcoming, and will have moved residents to a lounge or sitting room before our arrival. Usually we will get the same tea and biscuits as the residents, but in one place (now blacklisted!) we played for an hour and a half in sweltering heat and were offered nothing while they all got tea and cake: we pointedly helped ourselves to their water machine.