Hi Paul, I cooked up a converter for you, you can download it from here:
http://moi3d.com/download/3dmCurves2DXF.zip

Unzip this to some folder, then run the 3dmCurves2DXF.exe program.

It will ask you for the name of an input .3dm file, and then the name of an output .dxf file.

It will read through the input .3dm file and convert any curve objects it finds to Spline entities in the DXF file.

Please let me know if this works to transfer your curve data over.

- Michael