In WPF Viewer for Reporting Services till version 3.0.1 inclusive there was a problem with Multivalue combobox. Drop down wasn't shown on button click.
This happened because of the error in style of Multivalue Combobox.
It is necessary to add the following resources dictionary to the project:.
And add a reference to it in the Viewer resources:
<perpetuum:ReportViewer Grid.Row="3" x:Name="reportViewer">