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
Select open price and click Create Item or Done
Do not add any mods or costings to open price or by weight items
Add the new button to your menu on the Menu Editor page and press Save
On the iPad app press settings then Sync Menu and you're away!
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).