Because open availability options aren't associated with dates, they don't have an availability pane. Instead, you edit their availability from the option's edit page.
The number on the left is the current availability – how many of this option are available to be booked. The number on the right is the total availability – the max that should be made available at any one time. The total should only be changed if you are able to offer more or less of the option at maximum than you have been previously.
Change these numbers as needed, then click Save Changes.