What you need is a document stating who lived on your address. A quick round of google shows me that there is a very good chance you have to get it not from the Polish city council, but for some reason from the Belgian embassy in Poland.
Sounds a bit strange to me, but then again i've had more strangeness in Poland and we're practically neighbours (Nederland)
Actually, check document E401 mpips.gov.pl/gfx/mpips/userfiles/File/mps/koordynacja/E401.pdf from Polish gov.
On subscript 12 it states you get the proof of composition of your family from the Polish city council....