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You can also click on the column heading to easily see all of the items that are currently assigned to the page you are editing.
At this point the Item has been added to your store page, but there's one more step. The Item has to be "called" from the Template code on the Page tab.
The release notes or other documentation that comes with the Item will tell you whether the Item automatically modifies the Template code, or whether it has to be done manually.