Ok no blabla just let's go:
1- Make sure that u have the ubuntu repository updated cuz ffmpeg should be installable from apt-get !!
2- apt-get install ffmpeg libavformat-dev libavcodec0d libavformat0d
3- go to ur home/myhome and:
wget http://downloads.sourceforge.net/ffmpeg-php/ffmpeg-php-0.5.1.tbz2?modtime=1181869403&big_mirror=0
4- decompress tar -zxvf ffmpeg...
5- go to ur decompressed directory in /home/myhome/ffmpeg.../
6- phpize
7- ./configure
8- make && make install
9- go to ur php.ini file add: extension=ffmpeg.so
Too easy right !!!

Note: be sure u have enabled php-gd for generating video screenshot