If you have a veggie credit, your veggie credit will be applied when you add extra vegetables to your share beyond the standard $45 box when customizing your share. You can only use your veggie credit when your share customization window is open.
When you add extra vegetables to your share beyond the standard $45 box, the system will ask you if you are sure that you want to exceed the standard $45 box. Once you click yes, the system will apply your veggie credit to pay for the extra vegetables.
You do not have to use your veggie credit all at once, but you do need to use it by the end of the season, as veggie credits do not carry forward into the next season.
You can see your veggie credit in your CSAware account under "My Subscriptions":
The veggie credit has no cash value.