Hi Mike,
castlegarden.org has records of arrivals in the Port of New York - I did a
quick search and found 25 Kistners who arrived between 1840 and 1850, but
none of them were Conrad. There was a Johanne Wilhelmine who arrived in
1847 with a passle of kids, but she appears to be too old to be your
Johanna.
it's possible your ancestors landed in New Orleans and traveled up the
Mississippi by steamboat to St Louis. I haven't had any reason to look
into the New Orleans records in my own research, but perhaps they're
available through ancestry.com or in the LDS records as Max suggests.
good luck!
Art Dohrman
Huntsville AL