If you put "rex" into the search field, you will see that this is a quite common issue.
People have solved it in various ways, from installing the latest rex library from the Props, from installing the Reason demo, from deleting corrupt libraries or missing aliases to the library.
So stuff to try:
-
http://www.propellerheads.se/download/files/REX16Installer.dmg
- Install the Reason demo
- Moving the file from:
HD/Library/Application Support/REX Shared Library to
HD/Library/Application Support/Propellerhead Software/REX Shared Library
Let us know whether any of that fixes stuff for you.