You need to open the .obj file into pepakura designer. Then if you have the password save the file and it will automatically have it preset to save as a .pdo
From there you can edit it and so forth but if you just double click the .pdo file it will automatically open it in pepakura viewer so if you want to make changes to the unfold or change dimensions then you will need to right click the file and open it with Pepakura Designer not Viewer.
Hope that made sense...its uber early here and i stayed awake all night XD