http://www.kdenlive.org/forum/unable-add-new-clips-displays-message-saying-file-missinginvalid
I've got the same problem as this person but in OpenSUSE. Got it after trying to install Cinelerra.
I've tried the terminal code suggested in this topic even though it was suggested for OS X.
Of course it didn't work. Halp.

You probably have a problem in your MLT install. Which versions of MLT / Kdenlive do you have, and how did you install (where did you get the pakages, ...)