You can create open priced (misc) buttons to add to your menu by doing the following:
On your management site, create a new item and name it as you would like to see it on the receipt and on the button.
In the Sales price field enter "1" then drop down the icon to the right of Item
Select Open and click Done. (Do not add any mods or costings)
Add the new button to your menu and press Save
On the iPad app press settings then Sync Menu
The button should now appear on the app. to use the open priced button, press and add the sales price when you add the item to the bill. (This is entered in cents at the moment - so enter 500 for $5, 2100 for $21 and so on).