There are few reasons why after typing mocp you see this message:

• You haven't installed MOC or installed it improperly. Plugins are in /usr/local/lib/moc/decoder_plugins directory (the preffix could differ, this is the default location when you run ./configure without any options). If the directory doesn't exist MOC is not properly installed. Remember that mocp needs to be installed in order to work, compilation without make install is not enough because MOC doesn't search for the plugins in the current (source) directory.
• No decoder plugins were compiled. This isn't a fatal error during the compilation and installation, but prevents you to run mocp. The list of plugins to build is displayed near the end of the ./configure output. This is an example:

  ----------------------------------------------------------------
  MOC will be compiled with:
  Decoder plugins:    mp3 musepack vorbis flac sndfile modplug ffmpeg speex
  OSS:               yes
  ALSA:              yes
  JACK:              yes
  DEBUG:             yes
  ICONV:             yes
  RCC:               yes
  Network streams:   yes
  Resampling:        yes
  
  ----------------------------------------------------------------
  WARNING: since MOC version 2 the executable file name was changed to mocp!
  Please remove old moc binary if you have installed an older version.
  

  
  Look at the line that begins with "Decoder plugins:". If the list is shorter and you expect to see more plugins you should make sure all libraries (with -dev packages if necessary in your distribution) required for the given format are installed. Consult the README file and/or the rest of the ./configure output.
  
• You are trying to run MOC with plugins in different version than the mocp binary. This could happen if you compiled MOC and installed it in the place where it was installed by your distribution package. uninstall the package before compilation.