Doing the same thing now, this got me stuck also. In the WCF Workflow Services project, use the Add service Reference specifying the address of the service you want to call.
When you rebuild the WCF Workflow Services project you should see activities create for the service methods.
Place the activities on the designer and configure the parameters.
Here are some related links:
http://www.cloudcasts.net/ViewWebcast.aspx?webcastid=2521458133038050447