Activities are called using the Utility shape.  You will need to set the activity's usage setting to "Utility" on the Security tab of the activity rule form for it to be available for use in a flow like this.

Another standard way to call activities from flows is by going to your flow action rule and on the action tab for the flow action rule, specify an activity as the "pre" or "post" (or both) activity for the flow action.  /Eric