The earlier responses are incorrect.
It's Easy, see page 38 of the manual:
If only one battery is to be monitored, connect the violet wire to the blue wire. Also green & brown and orange & yellow.
At the back of the Link.
Also, don't forget to set F11 to OFF.
If you don't have the manual, Xantrex, who bought Heart out, has it on their website for download. Same material as the Heart manual - Xantrex knows nothing!
I'm a Happy Link 2000 user since 2003.