Details:
$ sudo apt-get install fuse-emulator-sdl spectrum-roms
$ sudo nano ~/.emulationstation/es_systems.cfg
And change the final ZX Spectrum section in the following line:
COMMAND=fuse-sdl %ROM%
Also, adding .tzx and .TZX to the extensions section will result in those file types appearing in the browser, too, allowing you to play multi-load games like R-Type.