Hiya Lloyd,
Okay first off,the DS linker and DS fire card are pretty much identical,so try the formatting option from the card by following the steps below,then post back here if that does'nt work.
# 1. Turn on power, enter DS/DS Lite menu first.
# 2. Hold "START+SELECT" and press "A" to enter DSLinker system.
# 3. A format menu will appear,select FULL FORMAT.
Try that and report back,if that has'nt helped then it may be a problem with the DS fire slot2 linking cart (the one for the slot 2 that the usb cable goes into)
If it turns out that your DS fire cart is dodgy,then i would suggest sending it back and getting a DSTT/cyclods/AK2 or soemthing that takes memory cards,as these tend to be a lot better and more supported than the carts with built in memory.