I am using iReport tool in conjunction with JasperReports 1.3.4.
I have a master report, which contain two subreports. One of these subreports has an embedded subrep
just set $P{SUBREPORT_DIR} as prompt and pass your value(the path to the directory may not be valid like c:\something.jasper instead of c:\\something.jasper)
$P{SUBREPORT_DIR}
c:\something.jasper
c:\\something.jasper