Mac OS X users can download Vice 2.3 which is a package of several related emulators. When downloaded, open the x64sc emulator and from the File menu select Smart attach Disk/Tape (or hit Command+O). Select Posts’s PRG file and before long you will see Jed’s blinking cursor. Adjust the emulator’s speed as necessary.
Of course you can still download the original Applesoft BASIC code and run it on an Apple 2 emulator. For OS X we recommend Virtual ][ which emulates the Apple ][, ][+, and //e. And if you happen to have an actual vintage Apple laying around you can use the audio file included in the source code package to load Jed onto it via the cassette port—just like the guys at Panic Software! See their blog post about it: An Apple //e, an iPad, and Jed.