please tell me how top add a watermark to a viedo.. :D

you put your video clip on track 1 and put your picture you want as the watermark on track 0. apply a transition called "screen". that's how I did a watermark on my video clips. you can also apply the pan and zoom video effect to the picture you want as the watermark also to move the picture around to where you want the watermark to be on top of the video clip.

As a side note when you add a title clip it auto adds the composite transition as that's how i do my text overlays on top of my video clips.

good luck.

thanks i think i did it but how can i save it as .avi?
it always prompt me to save as .kdenline.

how can i export it to .avi?

click on the render button, saving it saves the project for future editing or use in kdenlive. rendering it renders in your transitions and effects.