I have a KeyPass 2.x file (*.kdbx) that I created on a Windows computer using KeyPass 2.x. When I attempt to open the file on Mac OS X Lion using KeyPassX, I get the error "Wrong signature."
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.