Right now, LDS seems to have a major effort underway to revise its index by, amongst others, integrating the fast growing digitized (on-line) records. I wish that LDS would keep the old index of films and fiches accessible for some more time, until the new index works without any glitches. I guess that these big changes taking place were the reason why you could not locate the Findelhaus records.
The Findelhaus records are birth records of children born out of wedlock. The key information they usually provide is the parish of the mother where the official recording of the birth/christening took place. Since Vienna has many parishes, the foundling's birth record at the house of foundlings offers the prospect of saving a lot of search time.
To find the film numbers, go to the new home page https://familysearch.org/search
, click on CATALOG, then click SEARCH BY PLACENAME. Enter Wien in the LOCATION field, and select the first location the index offers you. This will be WIEN, NIEDERÖSTERREICH. Now click SEARCH BY SUBJECT, place, e.g., the word orphans in the subject field, click Search, and go from there. What you need are the films of the Findelbücher of year 1900. The following link might take you there directly: https://familysearch.org/search/catalog/139836