Parameter Variable Issue

Hello,

I'm creating an SSRS '10 report with a drop list parameter.

I have two data sets tied to the report:

Report3.rsd - This is the data set used in the reports

PMList.rsd - This is a data set tied to a stored procedure to create a drop list for a parameter @PostionDate.

Whenever I go to preview the report I keep an "An attempt was made to set a data set parameter 'PositionDate' that is not defined in this data set".

All I did in report3.rsd was add the following to create a variable.  Do I need to do any declarations and settings??

WHERE pm.POSITION_DATE = @PositionDate

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