Do you want to use PS as your primary editor?
Open iPhoto, select the iPhoto tab --- Preferences. You should see 'Edit Photo:' option. Click it and select Adobe Photoshop if it's there or a window should pop up so you can select the program from your Applications folder.
Now when you click a picture to edit it, PS will automatically open and allow you to edit the picture. When you save the picture it will be saved back into iPhoto automatically.
It this is too much, just export the picture from iPhoto, open it in PS, then drag and drop it back into iPhoto like Scarlet Fever said.