In order to create a shopping cart price rule that has free shipping you should do the following.
Go to Promotions>Shopping Cart Price Rules
Set the Rule Information tab up as you require.
Don't set any conditions in the Conditions tab.
In the Actions tab, set Apply to Shipping Amount to 'Yes' and set Free Shipping to 'For Shipment With Matching Items Only'.
You should now be able to test the voucher in the shopping cart.