I know this is an old post, but here is a solution anyway. We've seen this same problem when developing ASP.net pages with the Microsoft SQL Server 2010 Report Viewer. The font color displayed in the Multi-select parameters adopts the value assigned
to ForeColor in the ReportViewer. Make sure that the value that you assign to the ForeColor parameter in the ReportViewer is set to a color that is not White in order to see the values available in your multi-select parameter.