My Huawei U8510 doesn't recognise the card storage

[Rex Bunn]

Every time I buy a new app for my Huawei U8510 it tells me I have to remove some apps because there is no room on the device, in spite of the fact that my sd card has tons of storage. I've selected to option to save apps to the card, but this has no effect. How do I get the phone to save new apps to the card?