Some packages are in that directory and others have a subdirectory.
For the mosaic software you want the files
ftp://iraf.noao.edu/iraf/extern/mscredV3.2/mscred.readme # Instructions
ftp://iraf.noao.edu/iraf/extern/mscredV3.2/mscred.tar.Z # Source
(This information was extracted from instructions prepared by
Frank Valdes