The Enterprise Search Center requires that the Publishing feature be enabled. To enable the Publishing feature, follow these steps:

1. Click Site Actions, and then click Site Settings.
2. Under Site Collection Administration, click the Site collection features hyperlink.
3. Scroll down until you find the SharePoint Server Publishing Infrastructure feature, and then click Activate.
4. Move up to the Site Settings page.
5. Under Site Actions, click the Manage site features hyperlink.
6. Scroll down until you find the SharePoint Server Publishing feature, and then click Activate.