Code:
/usr/local/bin/mencoder '/home/xxxxxxxxxx/public_html/video/70_1.wmv' -o /home/xxxxxxxxxx/public_html/flvideo/11925866601469662619.flv -of lavf -oac mp3lame -lameopts abr:br=56 -ovc lavc -lavcopts vcodec=flv:vbitrate=500:mbd=2:mv0:trell:v4mv:cbp:last_pred=3 -srate 22050 -lavfopts i_certify_that_my_video_stream_does_not_use_b_frames
In ssh prompt type following command and see if that works.

From your post, u have not entered full path to mencoder when u try in SSH.