The bundle has been written to handle grouped output from ack. Each filename group gets a heading that displays the name of the file.

If you want to display the filename beside each match you'll have to rewrite a fair chunk of the output formatting code, but it shouldn't be tricky or anything.

Oh wow. You know what my problem was. I am using the "shiny" theme in TextMate. In that theme, the filenames are invisible! Switching to e.g. "default" solves the problem. Thanks for the hint!!

PS I was able to switch the theme on the Ack results panel (drop down) and it did not affect the theme used on the app's main screen (good).

Cool, thanks for letting me know :-)