KeyPassX says "Wrong signature" when opening a .kdbx (version 1.x) file
I discovered I could solve the problem by exporting the .kdbx file (2.x) to a .kdb file (1.x).

To do this, open the password file on a Windows computer using KeyPass 2.x. Select File -> Export, choose the file type as *.kdb and enter a filename. Then, you can open the new file on Mac OS X using KeyPassX.

Since my passwords don't change that often, I simply export a new copy for my Mac every time I modify the original on Windows.