Hi Tina,
If you can find the ship, then you order the Hamburg Passenger Departure List on microfilm from your LDS local Family History Center.
Or if you have the town name, this would really be important and you could order the parish records that serve the town.
I looked in my Germans To America (GTA) CD set for this family but could not find anyone close. The GTA gets it info from the Arrival Passenger Lists to U.S. Ports. (The GTA sometimes does not have every Geman passenger)
You should also ask someone that has a subscription to the Immigration database of Ancestry.com to see if they can find the arrival list and ship and date. Or if you have a local Family History Center near you, they have all of the Ancestry.com for free.
To find a Family History Center near you see:
http://www.familysearch.org/eng/Library/FHC/frameset_fhc.aspThe only problem is the Arrival Passengers List does not usually show the town in the old grand duchy of Mecklenburg-Schwerin, but just shows the Country.
So to get the town name you would need to order the other passenger list called the Hamburg Passenger Departure List. To do this you need to know the ship and aprox departure date. If you find the family in the Arrival List this will get you the arrival date and ship to be used to order the Hamburg Passenger List.
If not found in the Arrival List, then there is an index for the years 1856 to 1871 for the Hamburg Passenger List. You would need to order microfilm number 0884671 for passengers last names of HARUNG, Fridolin through KATZ, Salomon.
This is from the Hamburg List site at:
http://www.genealogienetz.de/misc/emig/ham_pass.html#15yearIf you find them in the 15 year index this will show the ship name and departure date. With this then you can order the "Direct List" for that date.
Let me know if you get stuck with any of this.
Of near the last resort, if you are certain they came in 1857 you could order the microfilm for that year (normally the Direct List) and look through the entire microfilm roll, name by name, but this may take you several weeks or months of hard work.
Another way (if you have some money to burn) is to send the info to Hamburg.de "Link to Your Roots".
http://ltyr.hamburg.deThey will search the Hamburg Lists for you for a fee, but may be worth it if all else fails.
Of note, I kind of doubt your Holtz came from Schwerin, Mecklenburg, Germany. Many people mistake the name of the old grand duchy of Mecklenburg-Schwerin as thinking the town of Schwerin.
For more info on the old Grand Duchy of Mecklenburg-Schwerin see:
http://www.genealogienetz.de/reg/ger1871.htmlThen click on the map showing Mecklenburg, this will get you some good info and History.
http://www.genealogienetz.de/reg/MEC/mec.htmlAnd also see the Mecklenburg-Vorpommern WorldGenWeb site at:
http://blacklake.biz/meck/My Website: Kruse Family from Mecklenburg-Schwerin:
http://freepages.genealogy.rootsweb.com/~fkruse/