This is a really nice set of hi-res MacBook PSD Mockups from Allan Ingwersen.
There are four mockups in total, each coming in at over 3,000px wide. In each layered PSD there is a smart object for using your own artwork, all you need to do is select the layer in question, and paste your artwork in there. The perspective will automatically be adjusted to fit the MacBook screen.
Please note these mockups are over 200MB, so it may be best to download these from your desktop/laptop.