Hiya kuczer1, welcome to T3G

As far as I'm aware the Sony Ericsson k800i uses the Memory Stick Micro M2 type of memory card, I don't think the MicroSD is compatible with that handset.

Looking at my 1Gb M2 memory card that I have for my Sony it's a different shape from all the Micro SD cards I have, they're about the same width at the bottom but the SD is smaller at the top, which would make sense as the contacts in your SE handset won't be meeting all the reader points in the phone, hence why you can't access it.

Where did you get the memory card from? Did you buy it separately from the phone? I'd recommend taking it back if you can and getting yourself the memory stick micro M2 type card.

Do let us know how you get on.

G