What version of Macintosh are your running? IDL 8.1 was designed to work with Mac versions 10.5.7 and 10.6.
See Help article for more information:
http://www.exelisvis.com/Support/Help...
Another option that may help you get IDL 8.1 installed is using the Mac-Unix installer (IDL 8.1 Mac OS X UNIX Install).