There is no grid. You can do a few things.
1) select the emitter name in the hierarchy so the position graph shows, then r-click in the position graph and select “reset”. This will put the emitter in the center of the stage
2) With the position graph selected, press the “N” key. This will bring up the numeric entry for the emitter position, and you can enter the pixel coordinates that you want.
Alan.
wondertouch