To preview a report in the Report Designer, press the Print Preview button from the Designer toolbar as shown below:

This allows you to choose a report section to preview.

Because you can nest many sections of various types within a single report, you have to select a section before actually previewing it. This way, the Report Designer knows what parts you want to compose on the screen.

Select a section from the Details list, then press the Add button to move it to the Selected Details list. Press the OK button to preview.