For the Premium Plan:
The 'Credits' orange box on the left of the screen, displays the total amount of credits available to reward people in your Store.
After you've rewarded credits to people, go to the 'People & Budget' tab to see the total amount of credits available for them to spend. Look for the 'Budget' column to find this information.
To see how many credits have already been spent, refer to the 'Spent' column on the same tab.
For the Basic Plan:
If you have pre-paid credits, they will be displayed in the pre-paid box. Otherwise, since you have joined a pay-as-you-go model, no credits will be shown.