The junction box is just a small plastic box (larger version available) with contacts for the wires of the cable. There is no cable/pug included.
You can use a ready-made master link cable with two plugs, cut it in two pieces and use one of these pieces to connect it with the junction box.
But a junction box is not necessary if you are able to solder or connect the cables in another way. If a max. length is exceeded, it might be necessary to shorcut 2 special wires of the cable then (that the junction box shortcuts "automatically"). Pls. use the search function of the forum to find detailed information.
The original master link cable is not weather proof but I think this will be no problem if you are using a cable canal. But if it is usually getting very cold or hot in winter/summer at the place where you live you should consider a cable by an alternative manufacturer. E. g. CAT5 network cables can be used as well although they have fewer wires than a masterlink cable (but not all wires of the masterlink cable are used). Network cables are also available as outdoor or underground versions.
See the documentation on Beotech for further information: http://www.beoworld.org/beotech/
The Powerlink-Cable distributes the left and the right channel to the speakers. The Beolab speaker itself is configured as right or left one by a switch. You can connect Speaker 1 to your audio/video device and Speaker 2 directly to speaker 1, so there is not need for a dedicated cable connection between every single speaker and the audio/video device. You could even use more than 2 Speakers in a Stereo-Setup. To realize a sourround setup with front speakers and rear speakers, one powerlink connection is used for the front speakers and a second (completely separated) powerlink connection for the rear speakers.