There is no way for this forum to host a sketch up file. If you have a site that can host it and then others can download it, like dropbox or some other file sharing site, then they should be able to provide you with a link if you find a button labeled "share" or something like that.

Once you have the link then there is an icon on the posting form for providing a webpage link. It is the second one next to the smiley face.
Try it and see if you can get it.
Good luck,
Jim Rogers