How did you install mencoder?
And FYI..... I highly recommend that you learn more about linux ASAP. I am not being mean at all, but so far, the questions you have asked are common linux knowledge. Get on google, type in "linux beginners guide," and take it from there. Linux wasn't made to be easy, also. Using linux requires some brain cells and IQ points, and we aren't here to hold your hand in every step that you take.
Cheers, and good luck!