Please try the following:
1. Launch Excel 2007
2. Click on the 'Excel Options' button in the menu from the Office Button on the top left corner of Excel
3. In the Add-Ins section, verify the AdCenter add-in appears in the Add-ins list.
4. In the Add-Ins section, select the 'Disabled Items' in the Manage dropdown list and click the Go... button
5. If the AdCenter add-in appears in the list, select it and click the Enable button