In InfoView (and only using Internet Explorer), you can drag the pre-defined "Last Refresh Date" cell onto your report, from the Chart and Table Types tab in the left-hand pane:

In Webi (using either Firefox or Internet Explorer), the steps are very similar, but you will instead find the "Last Refresh Date" cell on the Templates tab:

 

Or, if you already have a cell you want to put a formula in, the syntax is this:  =LastExecutionDate()