Rost family in Montana

I have been doing some genealogy research for my wife's family and have hit a wall. i have found a listing in the 1865 census that has listed the family and can trace them to Story City in Iowa from an 1880 census. i would like to find the date the family migrated to the US and how the family came by the last name of Rost. this is the link to the 1865 census from Norway - http://digitalarkivet.uib.no/cgi-win/webcens.exe?slag=visbase&filnamn=arkivverket/ft1865/f61927&postnr=2325&spraak= if anyone can help me out i would greatly appreciate it.