You search for the model you want. Ideally, filter your search on FBX, DAE, OBJ and 3DS files (these are what PlayCanvas supports):
Choose a model, click the 'download source' link at the bottom left of the 3D preview. That will take you to the site where you can download the model.
Once downloaded, just drag the fbx/dae/obj/3ds file into the Editor's Asset panel to upload to your PlayCanvas project.