It is in far southwest Germany. Go here: http://www.jewishgen.org/Communities/LocTown.asp
Enter Aselfingen in the text box, and click on "Start the search". It is a slow loading website. Then click on the "G" to see the location on Google Maps.
There is also a town spelled the same except for a double "s", Asselfingen. Judging by the microfilmed church records on familysearch.org, if your Great Grandmother was Catholic, she probably was from Aselfingen. If she was Lutheran, she was probably from Asselfingen.