I have seen people being able to convert the Sketchup models to a format which Unity can understand (but maybe that was in the Pro-version of Sketchup), but the problem usually is that Sketchup, due to its quite different modeling technology, introduces a lot of double geometry and either way one have to prune it in another modeling tool like 3ds Max, Maya or Blender.

To my knowledge, there is no easy method to convert sketchup models to format readable by Unity, apart from upgrading to the pro version.

Here is a discussion about how to convert sketchup models to fbx or 3ds models, in which you will find a method to convert dae files to fbx, using a program called fbx converter. See post #14. I did not try it myself but the poster seems to have good results, so you may give it a try :

http://forums.devshed.com/game-development-141/converting-sketchup-models-to-3dst-519710.html

Check out LightUp.co.uk which is a non-free visualization and lighting plugin for SketchUp (free and pro). I have used the free demo version and could export my SketchUp scene into Unity, including shadows. I've made some short screencast movies on Vimeo on how to export a lightmapped scene from SketchUp 8 into Unity 3: http://vimeo.com/19204786

Other solutions: the dreaded export to Google Earth and into Collada, which did work on my Mac, but not on a PC I tried.

Or use intermediate conversion software (Blender, FBX Convertor, Cinema4D (certainly not free) or others).

The PlayUp exporter for Sketchup exports models and level files directly to Unity. Can be found at http://PlayUptools.com

What you have to do is click and drag so every part of your model is selected and the edges are blue. Then click Edit and at the bottom you will see a number with the word Entities such as "18 Entities". Hover over that and you will see a option called Reverse Faces. Click on that. Then Export your 3d model as "dae" into your Unity Project folder. Everything should work!