1) Have you formatted the cards?
2) Try the cards in the card reader in your computer to see if they can be read.
3) Try the "switch off camera - remove and replace battery - switch on camera" trick with the original 128 Mb card in place. This will reset the camera's memory.
4) Note the type of card is important - not all cards can be used in a camera (especially some of the latest bigger ones) - check the type of cards supported by your camera.
I found this at the Nikon USA site (note that the 2 GB card is not listed - as said "Cards are not all the same" and use different drivers to operate them):
Which Secure Digital Cards
are approved for use in the Coolpix 4600/5600?
The Nikon Quality Department has tested the following cards
and approved their use in the Coolpix 4600/5600:
SD memory cards
SanDisk 16 MB, 32 MB, 64 MB, 128 MB, 256 MB, 256 MB*, 512 MB, 512MB*, 1 GB
Panasonic 16 MB, 32 MB, 64 MB, 128 MB, 256 MB*, 512 MB*,1 GB
Toshiba 16 MB, 32 MB, 64 MB, 128 MB, 128 MB*, 256 MB, 256 MB*, 512 MB
*High speed type of 10 MB/s or over
Other brands and capacities of cards
may work, but Nikon cannot guarantee their operation. Check with the manufacturer of the third-
party card for compatibility information. Nikon recommends keeping one of the approved cards
available for troubleshooting.