Try not using the entry in the list to add you objects.
i.e. Don't use ........\h_w_walls2
Instead use the Add icon in the top left corner of the Pb Manager......then navigate to your Addons folder and choose the pbo file containing your objects.
i.e. h_w_walls2.pbo
The difference is there is no '\' in front of the name.
Once you have done that, you will find h_w_walls2 listed a second time nearer the bottom of the list, this time without the preceding '\'.
Hopefully this will fix your problem
Planck