Answer by Mortim

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