Adoptions were handled by the probate court. Those records are now held by the Area Research Center of the Wisconsin Historical Society at UW-M. See:
http://arcat.library.wisc.edu/cgi-bin/Pwebrecon.cgi?PAGE=bbS...
Search for: Author = Wisconsin. County Court (Milwaukee County)
Title = Probate case files
The catalog record says "Some unprocessed accessions contain information that is confidential under s. 51.30, 1995-96 Wis. Stats. (guardianship and insanity files) and s. 48.78, 1995-96 Wis. Stats. (adoption and juvenile records). Researchers should contact the reference archivist for information regarding access." So you may not be able to access those records, if the adoption happened in Milwaukee. If the adoption happened in Minnesota, you'll have to check the records situation there.