Hi Sean, In case you did not know, I should start out by mentioning that IDL 8.3/8.4 are not currently supported for use on Mac 10.10. IDL support for Mac 10.10 will become available at the next release of IDL and ENVI, which should be later this year. With that being said, there are still plenty of people who have gotten IDL 8.3/8.4 to work on their Macs. It sounds like you may have found this already, but here is a link to a help article which provides some additional information and links to help articles which discuss other topics in more detail. http://www.exelisvis.com/...X-1010-Yosemite.aspx The third article included in the link discusses uninstalling and reinstalling X11/XQuartz which may be a good idea as well if your error persists. It may not hurt to re-boot your machine if you have not done that since X11/XQuatrz was installed.

Also, if the product installation process itself is hanging, you may wish to try downloading a fresh copy of the installer package file, in case the one that you have is corrupted.