Bruce,
Try opening it once with ctrl+click and answer the following answer withYES. The DMg most probably is okay, this is a new OS X feature.
Joachim
Hi Damien,
I tried your .dmg file on my iMac running OS 10.9 (Mavericks). It
didn't work. The error message is "Pharo.app is damaged and can't
be opened." A second download and retry gave the same result.