The highest StuffIt Expander version for System 7.5 would be 5.5 ( you may want to look for it at
ftp://ftp.allume.com/pub/archive/ ).
Mac application files on the Internet are encoded BinHex (.hqx) or MacBinary (.bin) for protection. Files may be in a merely compressed form (.sit), too. If you are using a PC for the download, do not decode/decompress until on a Mac. Once on the Mac, do not double-click on the file (instead, drag the .hqx/.bin/.sit originating from a PC onto the Stuffit Expander for Macintosh program icon).
As Jim also indicated, one of the difficulties involved is that something like StuffIt or StuffIt Expander is needed to decode a newer StuffIt Expander from the Internet. This is, of course, especially problematic when not even an older version of the utility is available. Strangely enough, the easiest way to solve this is sometimes via a PC. A file
here, called MACDISK.EXE, can be downloded onto the PC. Next, prepare an empty PC-formatted 1.44 MB diskette via the FORMAT A: command in DOS or the "Full" formatting option under Windows. Run the MACDISK.EXE program on the PC. Follow the on-screen directions. The result will be a Mac-formatted floppy, including a ready-to-use StuffIt Expander 4.0.1 installer.
As far as the CD is concerned, try a plain CD-R instead. A disc from a PC in an ISO 9660 format should work. Make sure that the appropriate CD-ROM extension and the required helper files have been installed on the Mac.
Jan