In expressions, refer to parameter values using Parameters!param.Value
.
In your example, add an expression similar to this to a textbox in the header:
="Report run from " & Format(Parameters!dt_begin.Value, "dd-MMM-yyyy")
& " to "
& Format(Parameters!dt_end.Value, "dd-MMM-yyyy")
All it's doing is formatting your parameters and adding them to a larger string.