Just go to File - Export - 3D Model. It will be exported as a .dae file, which can be uploaded direct to Shapeways. You may want to check your file. Get yourself copies of http://meshlab.sourceforge.net/ and http://www.netfabb.com/basic.php they are both free. Meshlab will read your .dae file. you could use some of the filters to repair it if need be. From here export it as a .stl file. Open the .stl in Netfabb it is more user friendly, you can check, repair, measure and scale your model. You can upload either the .dae or the .stl version.