The plugin chain requires an input, i.e. a source of video frames to process. The input to the plugin chain comes from a video file (AVI/MPG), a still image (BMP/JPG/GIF), or a special type of Freeframe plugin, called a source plugin. Source plugins differ from effect plugins, in that they don't take any input, they only create output.
Note that FFRend needs AviSynth to play MPEG files. If you try to play an MPEG without AviSynth, FFRend displays an error message asking you to verify that AviSynth is correctly installed. AviSynth is free software; you can download it from http://avisynth.org.
To use an input video or image file, choose File/Video/Open, or drag the file from Windows Explorer and drop it onto FFRend's main window, or drag it from the File Browser. To use a source plugin, load it into the plugin chain, in the same way as an effect plugin.
Note that you must have a video or image file open, or have a source plugin in your plugin chain, otherwise you won't get any output. If you have both a video/image and a source plugin, the source plugin wins: the video/image remains hidden unless you bypass (or delete) the source plugin.