RJ11 is just the little flat plug on the end of the cable that plugs into your modem click here.
They seem to be suggesting that you make up a long cable direct from the master socket to your modem, instead of going via the extension sockets. Don't really think this will make much difference to be honest, as an extension is just the same cable with a socket on the end, although you will be cutting out 1 connection.
If the ADSL light will not remain steady then the issue is with the line (BT) or ISP. Unfortunately BT will not look into line issues unless pressed by ISP (wannado). You were told before signing up that your line was suitable (even at the lowest speed) and therefore the ISP should be responsible for sorting it out or you can claim your money back.
Have a search in the left search box there have been several other threads on the same issue I'm sure one of them describe how to 'push' the ISP and BT.