Problems with imports with sketchup 7

I use sketchup 7 Pro and I have a question about it, when I make my rollerocaster a 3d object in the no limits construction kit, I save it as a 3d in the no lmits object forlder. I open up sketchup and click import, got to no lmits objects, find the track object and open it, then a couple of seconds later(when the object is loading) somthing pops up saying file not found or invalid!
PLease help, it would be much appriciative!

You have to export it as a .3D file

no .3Ds file or did you forget the S? but I'll go and see.

Thanks alot it worked!!!