I have installed the latest version of Dia on my Mac to test and used the diainstaller to add other shape files. In attempting to use the gradient tool, the svg files being referenced don't appear to be recognized (at least that's the returned error – file format not recognized). I checked the .dia directory and it appears all is there in the shapes/gradient folder. I did notice however that when viewing these folders from Finder that the diashapes files were not present. I'm not clear if this is a Mac OS X referencing issue (I.e. From the 'contents' folder of Dia to the .dia directory). I have checked the path in the shape file for each gradient block and all appears correct. I also made sure that Inkscape could view the .svg files.
So I'm not sure what's happening here and I haven't found a single report of this, so thought I'd post.