How can I add a 'date last refreshed' to my report?

With your report open in DESIGN mode, click on the Report Element tab > Cell sub-tab > Pre-Defined > and select Last Refresh Date from the drop-down menu. You can then add the cell anywhere you would like in your report by clicking on the report. Or, if you already have a cell you want to put the 'last date' formula in, the syntax is this:  =LastExecutionDate()