There have been some reports of problems with newer Macs (running 10.9 and later) where a message stating that the RTIBuilder download is damaged appears.
This is a known problem that started with MacOs 10.9 and later, because Apple changed their security checks.
We do not believe there is anything wrong with the RTIBuilder download. As a small nonprofit company, we haven't paid to be in the Apple Developer program, and so the content is unsigned. Also a small script runs when you install the software (it basically just sets up a path to the software) and this is what is causing the error message. Note that the identical package on MacOS 10.8 and earlier will not generate this message.
You need to change your security settings (found in the system preferences) to allow software downloaded from anywhere to run, and then try it again. You can then change the settings back.