Nikon D300 and Sd card that will fit in Expresscard slot?

The Nikon D300 uses CompactFlash cards, which (as you've discovered) are larger than the MBP's Expresscard slot. There are CompactFlash card readers that will plug into the Expresscard slot, but the card itself and much of the reader will project out a couple of inches from the body of the computer. Such a reader is expensive, bulky, fragile, awkward to transport, and quite vulnerable to accidental damage when it is in position for use. Your other choices are (1) a USB card reader (I have a $10 multi-format reader to handle the five different card types that my cameras and other devices use; it's more compact and much more durable than any ExpressCard CompactFlash reader I've seen) or (2) connecting your camera with the card inside it directly to your computer. For the latter procedure, see your camera's instruction manual.

There is no physically smaller card format that you can use in the D300.

Aha! Check this out:

http://the-gadgeteer.com/2005/06/10/eagletecsd_to_cf_ii_card_adapterreview/

This would enable you to use SD cards in your camera. ExpressCard 34 readers for SD cards are fully housed inside the ExpressCard slot, and can stay there all the time if you have no other use for the slot. I believe an SD card will protrude a bit from the computer case when it's in the reader, but much less so than a CF card-and-reader combination does. And when the reader's not in use, you can leave it in place, where it's protected from damage and takes up no space you weren't carrying around anyway.

I may buy one of these to use in one of my own cameras, since I already have several large SD cards for use in a digital audio recorder.