Here is a tip for displaying data from data-sets into the Page Header/Footer of a SSRS report. Traditional the hacky way of performing such tasks is to have a hidden text-box in the body of the report with the respective data-set field value. You can then get the value of this text-box in your page header doing the following: =ReportItems!txtBox1.Value. The major issue here is that the data is not available on multiple pages, so if your report has a large data set you will notice the value will not be shown on each page.
A better solution is to create a new internal report parameter that references a field value from the selected data set. You can make the report parameter internal and reference it in your report header by doing the following: =Parameters!myParameter.Value.
Its cleaner and is available on every page.