Try SharePoint Designer 2010 Technical Preview!
The feature you are using requires SharePoint Designer 2010. At Release to Market (RTM), this page will be the free download page for SharePoint Designer 2010.
TAP Customers Download
During Beta1, TAP customers can install the Technical Preview of SharePoint Designer 2010 by navigating to one of the following Connect pages: English x86; English x64; Japanese x86; Japanese x64.
Internal Microsoft Users Download
You can can install the SharePoint Designer Technical Preview through this internal page.