I just tried it and it's working for me. It could be a setting on your computer. You might try restarting your computer and trying again.
Once you find your campground, you should be able to enter dates on the left hand side. To the right, going across the screen you should have the following tabs:
Facility Details Facility Map Site List Date Range Availability Visitor Photos
Once your dates are entered, the Facility Map should show you an interactive map with the campsites - the blue icons are available campsites; the Site List will list all the sites with size and availability; the Date Range Availability will show you each site with an A (available), R (reserved), or W (walk in) - it will show you the 2 weeks around the dates you plugged in, but you can click on the arrows to go further out or look at earlier dates.
Hope that helps.