Take a look at this thread: Re: BI Publisher- Display Parameters?
You could also include a new data model with a sql query like select parameter1, parameter2 from dual and specify concatenated sql data source (using 10g version) at the data model level and the parameters will be included in your xml data source and then you can just use them as needed.
In 11g, as along as you select (check) the option "xml output options ->Include Parameter Tags" under data model properties, the parameters will be included in your xml.
you can include parameters in xml output by property in data template
include_parameters - Indicates whether to include parameters in the output.
for more info look at "Properties Section" in Oracle Business Intelligence Publisher Report Designer's Guide