I have a SanDisk 256 MB SD card, and I love it. There used to be some major issues with them, but not as of recently. I would recommend inserting the card in a SD card reader attached to your PC, and see if you can see it this way. If not, you may have to get a replacement. If you can see it in a PC, format the card with the FAT16 file system, and then reinsert it in your Axim. Also, if there is any data on your SD card, copy it to your PC before formatting, as a format erases all data on the card.
If you see it and can get it formatted using a PC card reader, let me know. I use a PPC utility to format my SD card to maximize storage space and speed. Since I've used this util to format, I've not had any issues with my SanDisk 256 MB Card
