Calendar problem - can't choose file

Hi! I just upgraded to a new Mac and Mavericks - a big jump from Snow Leopard! However, I'm finding that I can't re-create some of my previous calendar events - specifically, ones I have set to "Open a file" when they occur.

(These events launch an Automator action that does a backup of a client's website - so it's important that they keep working!)

In the event's popup info window, I click on Alert and choose Custom..." Then I switch from "Message with Sound" to "Open File.." Finally, I change "Calendar" (just below "Open File...") to "Other...". At this point, I expect a file-selection dialogue box to appear. But... nothing happens.

This was working initially, at least for some events, as I made some adjustments after switching to my new Mac. But now, it doesn't seem to work at all.

Anything I can try?

Adam