On Windows 2000 and XP, this is a folder under c:\Documents and Settings\<username>. On Windows Vista and Windows 7 this is a folder under c:\Users\<username>. This folder may typically also be accessed in Explorer under My Documents\DPlot.
On previous versions of Windows the referenced file may be found in the folder where DPlot is installed. If you used the default installation path of the setup program this is c:\Program Files\DPlot.