Select "Data Preparation->Flow Accumulation-Single Flow Direction".. Specify the parameters and the output file.
Output: Flow Accumulation
Flow accumulation is an indirect way of measuring the drainage areas (in units of grid cells).
Method: Singe Flow Direction
A single flow direction algorithm assumes that subsurface flow occurs only in the steepest downslope direction from any given point.
Flow Direction File
A raster of single-flow direction file of the area.