I have one of these on my 1994 XT600E. The red wire (+) should be taken to any switched positive, the black (-) to any earth.
For the most accurate readings, connect the + wire as close as possible to the ignition switch, so you aren't seeing voltage drops through long cables and iffy connectors. The - wire can go directly to the battery negative, or to any sound chassis earth. I have the + on mine cut into the wire from the ign switch about an inch from the switch, and the - goes all the way back to the battery.
You can safely wire it to the same connectors as the power socket, as the voltage indicator draws only a tiny current. I find the device very useful. I was getting a solid green light which occasionally flickered to amber, and told me that my ign switch was going bad. And when the reg/rec goes again (as it will), hopefully I will know about it before it leaves me stranded.
If you need any help, give a shout.