formatting the MMC ?

try menu-extra's-memory card. in the options list there is an option to format

There should be a icon in on of the main folders called 'Memory Card' or something like that, you can format it there.

Otherwise, read this:
http://www.allaboutsymbian.com/forum/showthread.php?threadid=17507

-kris

I found the icon.
It also says that it formats the MMC.
But after formatting every file is still on the card and it seams that nothing happened.

Use a card reader

Format it as FAT