The bug in the OpenForm document, we got that fixed in 5.1 - here's the user guide info for that:
OpenForm
Opens an XSN form, using an XML file if provided. Not browser compatible.
Parameters:
o path: Path or URL to XSN to open.
o data: Path or URL to XML to open. If /path is also provided, /data will be opened with that template.
o dsname (optional): Data source for node indicated by /xpath parameter to provide data for template indicated by path.
o xpath (optional): XPath for node with data to be used with template provided by /path parameter.
o new: (optional) true|false - defaults to true. Set to false to open existing XML. If false, must include /data parameter.
Example:
o OpenForm /path=http://testserver/site/FormLibrary/template.xsn
o OpenForm /path=C:\Users\ernestom\Desktop\qRules\Test forms\ExpenseReport.xsn /data=http://shp2010-test2/ExpenseReportTest/2012-09-28.xml /new=true
Additional observations:
o The OpenForm command is only availble in client forms.
o Either /path or /data must be specified.