I realize that Location and GroupID attributes mentioned in SDk are not correct: http://msdn.microsoft.com/en-us/library/bb802730.aspx

The values you specified are correct.

I was able to display a link with following XML, which is pretty similar to yours:

    <?xml version="1.0" encoding="utf-8"?>
    <Elements xmlns="http://schemas.microsoft.com/sharepoint/">
      <CustomAction  GroupId="SiteCollections" Id="CheckAllaboutmoss" Sequence="100" Location="Microsoft.SharePoint.Administration.Applications" Title="Check Availability" RequiredAdmin="Delegated" >
        <UrlAction Url="/_admin/createsite.aspx"/>
      </CustomAction>
    </Elements>

**Please make sure following:
1. You can use Scope = Web or higher (Web makes sense)
2. Make sure you have AutoActivateInCentralAdmin="TRUE" in the FEATURE element of the feature.**