It seems that ACL for coupons is not defined. Please merge the following snippet into ../Mage/Checkout/etc/api.xml../Mage/Checkout/etc/api.xml. It should make proper ACL item available in the the admin panel.
<config>
<api>
<resources>
<cart_coupon translate="title" module="checkout">
<acl>cart/coupon</acl>
<methods>
<add translate="title" module="checkout">
<title>Add coupon code for shopping cart</title>
<method>add</method>
<acl>cart/coupon/add</acl>
</add>
</methods>
</cart_coupon>
</resources>
<acl>
<resources>
<cart translate="title" module="checkout">
<title>Shopping Cart</title>
<coupon>
<title>Shopping cart ability to set coupon code</title>
<add>
<title>Add coupon code for shopping cart</title>
</add>
</coupon>
</cart>
</resources>
</acl>
</api>
</config>
