I'm new to the CS6 version so am not sure what you mean by linking. I simply place the image and fit content proportionally to the frame, the same as in CS4. At 4x5 the image is good for 546 ppi, Photoshop tells me. Obviously the screen can't display 546 but it's looking more like about 50 ppi.
The procedure is exactly the same as it was in CS4. It sounds like the file was moved or modified after placing. Was this a file created from the beginning in CS6, or an older file you are updating?
The layout file, minus photo, was created in CS4. After opening it in CS6 I then placed a photo and got the jaggies. Did not move the photo after placing it. My guess is that CS6, being a new installation, was settling in. (That's a technical term.) In any event it's working smoothly now, no need for an extra linking step with other photos that I'm placing.
Thanks for your help, Peter.