In blender, at the top, click on file, then click on import and then find the file extension that the file has that you need to open. You will then need to know navigate to where you have the file located.
Blender can NOT open a .pdo file. You will need to convert it to something else that blender can open. You can always open the file in pepakura and as long as the file is already unfolded, you can go to file, export and at the bottom of the list, it will let you save it as a .obj file which blender CAN open.

Hope that helps.
However, I THINK you need to have bought a license key so that you can fully use all the options in pepakura. I'm not 100% sure.