The answer I got from Todd
(in case anybody whose Mac originally was set to another language preference ever has the same question)
RSD is set to work in English. Your default settings are for another language. To get it to install, set OS X to English by dragging English to the top of your International Preferences Pane in System Preferences.
Go to the Files to Install from the Ready-Set-Do_1_4_Demo.zip file that you unarchived. Locate the folder that says "English". Change it to anything else - like English1 or something. Then run the installer and RSD should install just fine.