(Read about PmDefaults PmDefaults.)
Find the latest file of the form "pmdefaults-mac-nnn.zip" under portmidi in the Sourceforge PortMedia Project Files page. (It may not have the latest version number, so you might have to search a bit.) This file contains an application bundle for OS X.
On Snow Leopard, you need to open the application properties and check the box that says "Open in 32-bit mode".
You will also need the JavaVM since PmDefaults is a Java application.
Alternatively, you can follow directions for building PortMidi from source.