Komposer is a KDE ap. Since you didn't specify which vl version this is, I have to assume because of the hardware that you listed that you are running either the Light or STD version, which none of them have KDE on them.
With that said, the xfce or icewm (in the case of light) may not show komposer. You can always open a terminal and execute
komposer to fire up the application. If it works, it's just a matter of creating a desktop shortcut or a panel launcher.
As far as your partitions go, they must be mounted before you can browse them... I'm not sure PCManFM can do that on its own.