To calculate streamlines from your flow please refer to the MATLAB file streamline.m (type help streamline at the command prompt). This file calculates streamlines from a given set of starting points, and to ease the plotting MatPIV comes with a file calles mstreamline.m which will create a set of starting points on all the edges of the flow field.
The file streaml.m, originally included with MatPIV has been discontinued and will no longer be updated.