I have my slic3r set up so that the center of the part will be at the origin of the XY plane.

For pronterface to center these objects, I needed to go to Settings–>Options, Printer settings tab, check curcular build platform, and enter these settings:

Adjust Width, Depth, XY offset to match the size of your print area (slightly smaller than your print bed). The Height is from my MANUAL_Z_HOME_POS setting in Marlin/Configuration.h