A plugin/crate was declared but cannot be found. Erroneous code example:

#![feature(plugin)]
#![plugin(cookie_monster)] // error: can't find crate for `cookie_monster`
extern crate cake_is_a_lie; // error: can't find crate for `cake_is_a_lie`

You need to link your code to the relevant crate in order to be able to use it (through Cargo or the -L option of rustc example). Plugins are crates as well, and you link to them the same way.