On my MacBook Pro, how can I open a picture in Photoshop that is in my iPhoto library?
You can set Photoshop (or any image editor) as an external editor in iPhoto. (Preferences -> General -> Edit Photo: Choose from the Drop Down Menu.) This way, when you double click a pic to edit in iPhoto it will open automatically in Photoshop or your Image Editor, and when you save it it's sent back to iPhoto automatically. 

This is the only way that edits made in another application will be displayed in iPhoto.