Well, it should be really simple to open up a png in photoshop. Fire up photoshop, and go to file->open. On that dialog, make sure that the option for 'Files of type:' is set to either 'All Formats' or 'PNG', and then just browse to where you have your png's saved and you should be able to open them no problem.

That's the absolute basics, and my apologies if it's too basic. I've never had any issues opening png's in cs4, so I'm guessing your problem is probably to do with 'Files of type:'