You don't use a skp file to upload to the warehouse, you have to use a kmz file.
There are a few ways you can get a kmz file to upload your model. The easiest would probably be to upload straight from SketchUp using the Share Model tool in the Google Earth toolbar. This will automatically generate a kmz file to upload, as well as bring up the window to upload your file.
Another way would be to use the File > Export > 3d Model, and save as a kmz file. An unnecessarily complex method would be to use the Place Model tool, also in the Google Earth toolbar, then save use File > Save > Save Place As... from Google Earth to get a kmz file. Once you have a kmz file you can then go to the 3D Warehouse upload page to upload that kmz file.