DC circuit schematic has both positive and negative supply as well as ground?
As PlasmaHH said, 


  GND is the arbitrary point in a schematic we assign 0V to.


You then need two power supplies, each one generatong 4.5 V. You connect the positive terminal of the first one to the +4.5 V node in you circuit and the negative terminal to GND. Then, you connect the positive terminal of the second one to the GND node in you circuit and the negative terminal to the -4.5 V node.