Andrew (Anders) and Martha Oleson

I am looking for information on my great grandparents - Andrew Oleson was born about 1815 and Martha was born about 1817. They emigrated to Minnieska, MN from Norway in about 1852 bringing with them 4 children, Ole born about 1845, Bertha born about 1847, Albert born about 1849 and Johanna born about 1851. These dates are estimated ones as taken from the MN censuses. I have all the censuses on them in the United States, but cannot seem to locate them in Norway in the censuses there. Does anyone have any information or suggestions that would be helpful in pinpointing them in Norway?