Many of the 3d models you find in Google 3D Warehouse and Thiniverse will be too complex for folding.

This article describes a technique for reducing the number of polygons in the mesh.  It involves downloading a free program called MeshLab (available for OS X and Windows).  Follow these steps:

1. Download the model in SketchUp format from the 3D
2. Open the model in Rhino, Export the file as a “.obj” file (just as you do for opening it in Pepakura)
3. Open the .obj file in MeshLab and follow the instructions in the above article
4. Save the new/reduced mesh as “.obj” and continue wtih this file in Papakura

With this technique, I was able to take this model of a hand from the Google 3d Warehouse

and reduce it to this:

 Which I was then able to unfold with Pepakura, far more reasonably: