There are several things that can cause curling on Barberries, but the most common two in my experience are aphids and stress. Aphids should be fairly easily apparent if they are causing the curling. You can control them with insecticidal soap.
I think this may be some form of stress symptom. It may have been caused by heat, overwatering (what happened to some of mine), underwatering, or something else. The best thing you can do for these plants is to get them in the ground. Once they are planted, they should improve, although the curled leaves may remain that way. Judge by looking at new growth.
When I planted one in my yard, it was too wet, and the leaves curled. They remained that way all year, until summer when the ground dried a little. The new growth after that wasn't curled. Ever since then, the plant had been great, except for powdery mildew one time, which also curled the leaves. (I removed it in '06 because of the size.)