If you are not able to find the Addon Domain icon in cpanel for a particular account which is using Rvskin, then check whether the account have disabled addon under the option Modify account in WHM

"Quota Addon = 0" If this value is 0 it will not show the Icon for Addon domain in cpanel.

If it is true please verify package in RVSkin Manager >> Config >> Account-Feature Manager

If the account do not have any package created then create on and assign to the account this will fix the issue.