mount a encrypted external hd


i have an external toshiba mk1637gsx 160Gig hd

i would like to mount it

formated in ext4

/dev/sdc 149.05 Go
/dev/sdc1 156.00 Mo (ext4)
/dev/sdc2 148.89 Go Linux LVM (encrypted)

in kde, when i click on sdc2, i need to enter a password… after i get a message

org.freedesktop.UDisk.Error.Failed: Not a mountable file system

how to mount my encrypted file system?

any idea?