There are two approaches in order to actually get report parameters to work.
Approach one: when you create your underlying dataset, you explicitly define the parameters in a where clause of the statement by putting a @ symbol in front of the identifier (name) of the parameter. For example:
select employee_name from Employees where employee_id = @emp_id
Here @emp_id states it is a parameter and when you create such dataset SSRS actually performs two actions - first it creates a parameter (you can find in Parameters Tab in BIDS) and second - it binds the newly created parameter with the parameter defined on a dataset level (when you define a dataset, there is a Parameter tab, where such definitions are created
Approach two - you first create a parameter and then reference it in the dataset - if you did this, you must open your dataset properties, go to Parameters and explicitly define binding between parameters in the report and the parameters in the dataset (again, parameters in the dataset must be starting with @symbol)
My reccomendation is to use always approach one as at most of the times it does most of the work for you and all you need to do is customize the automaticalyy created parameters.