The Mix Core gets installed in the lowest numbered slot which is the bottom-most slot for the Magma PCI-6DR, followed by Mix Farms,d24, Mix I/O, DSP Farm, Samplecell (in that order).
However, I would suggest you download the PT 5.0.1 Expanded Systems Guide at:
http://www.digidesign.com/support/documents.html
Brent