Use the Encore Advanced Search (you can get to this from the main webpage by clicking Books & More > Catalog > Advanced Search)
Enter * in the Keywords field
Select DVD from the Format dropdown
Click the Search button
Alternatively, you can enter * in the search box and then select DVD from the Format facet on the left side.
If you want to filter the results for just children's DVDs or just adult DVDs, you can select one of the choices from the Collection fact on the left side.