When I select a core in the main CORE Generator window, the only action available is View Data Sheet. Double-clicking the desired core will open the data sheet for the core.
From Project Navigator Project -> New Source, a user might see the data sheet for an IP core open instead of the desired IP core customization GUI.
This occurs if the targeted IP is not available for the specific hardware device selected. The most common case is when the selected device is too small for the core in question.
Example:
If the target device is set to xc4vfx12 and the Fibre Channel v2.0 core is selected, View Data Sheet is the only option available. Open the data sheet and notice that the core will fit in 4VFX20 or larger Virtex-4 devices.