Once dBpoweramp crashes, you're presented with a system dialog:

Appearance may vary with Mac OS version. Please click the "Report..." button as indicated on the picture above.

In the Problem Report dialog, click "Show Details":

Finally, plaintext description of the crash appears-

Select all the text, copy it and post on the forum, or send to one of the developers if instructed to.
To quickly select and copy text, use Cmd+A (select all) and Cmd+C (copy).