Can't do subtraction with Boolean modifier I'm trying to get my hands into Blender after months using Sketchup and facing the limitations of that last one (subtracting shapes is limited to PRO and even in pro, it is still quite limited).

I first tried to export a collada file but then I read that maybe my objects are not plain. 

So I did what I usually do when I check my stl files for 3d printing, I repaired them, and then re-exported them through collada and Imported then into Blender.

Unfortunately, I still can't do the subtraction. 

Surely somebody here will tell me what's wrong? Maybe the whole approach? 

I attached the blend file (2.6 version) so some charitable person could have a quick look and spare me light years of trial and error?

https://drive.google.com/file/d/0ByBlKzf_FnbIaEdKaUhrZk11Nlk/view?usp=sharing

Resolved. Thanks to @Mr Zak for his help! (See second comment)
Your mesh has to be non-manifold. I had troubles with this as well. Removing doubles it typically a good way to start achieving non-manifoldness. There are enough tutorials out there and somewhere in the documentation there's a hint, that your volume has to be non-manifold for the boolean stuff to work properly.