Newly added scopes at SSP level don’t show up in Search Scope dropdown

Ok, when you add a scope on SSP level it doesn't show up in the Search scope drop-down for the sites.
This is because SharePoint gives us great flexibility and control over the search features. So any new scope that we add at SSP level will be available to all the site collections (as unused scope) but it needs to be enabled.

Steps to do this:

1. Open the root site in the Site collection and open the Site Administration page (Site Actions --> Site Settings)
2. Click the 'Search scopes' under 'Site Collection Administration'
3. The newly added scop is listed under the 'Unused Scopes' group
   To make it available in the search scope dropdown for the site collection we need to make this scope available
   

4. Click/Edit 'Display Group: Search dropdown' scope group
5. Check the 'Display' checkbox for the scope you added at SSP and click OK
   

6. Repeat these steps for 'Display Group: Advanced Search dropdown' scope group if required

Now the newly added scope will be visible in the search scope dropdown on the site collection.

You can also remove certain scopes from the search dropdown by unchecking the 'display' box. If you do not want to make a scope available to the site collection, make sure to remove it from all the display scope groups viz. Search dropdown and Advanced Search dropdown.