If you use Skeinforge and Pronterface with the configurations provided it will print in the correct place (0,0) which is the centre of the bed on Mendel90. If you use other hosts and slicers you have to configure them to slice at 0,0 and display 0,0 in the centre as they often default to 100,100 being the centre. There are some examples near the end of the manual.
BTW, there is a forum dedicated to mendel90. [
forums.reprap.org]
[
www.hydraraptor.blogspot.com]