How to add acl in custom module in magento

A general explanation:

ACL for admin menu

To define the ACL for a custom admin menu entry, copy everything below adminhtml/menu to acl/resources/admin/children and remove the <action> nodes.

Example: What to copy

To actually use the ACL you have to add the following method to your controller:

protected function _isAllowed()
{
    return Mage::getSingleton('admin/session')->isAllowed('ENTER RESOURCE IDENTIFIER HERE');
}

The resource identifier is base on the node names below acl/resources/admin/children, skipping following children nodes.

Example: Resource identifiers

ACL for system configuration section

To define the ACL for a system configuration section, the following has to be added below acl/resources/admin/children:

<system>
  <children>
    <config>
      <children>
        <my_configuration_section>
          <title>My Configuration Section</title>
        </my_configuration_section>
      </children>
    </config>
  </children>
</system>

where my_configuration_section is coming from system.xml:

<sections>
    <my_configuration_section translate="label" module="my_module">
      ...
    </my_configuration_section>
</sections>     

---

Specific to your question:

In your case, that means, adminhtml.xml should look like this:

<?xml version="1.0" encoding="UTF-8" ?>
<config>
    <acl>
      <resources>
        <admin>
         <children>

           <customer translate="title" module="adminhtml">
             <sort_order>100</sort_order>
             <children>
               <set_time>
                 <title>Seller List</title>
               </set_time>
             </children>
           </customer>

          </children>
        </admin>
      </resources>
    </acl>
</config>

Create adminhtml.xml at Webcreon/Seller/etc where you need put your code

<?xml version="1.0" encoding="UTF-8" ?>
    <config>
        <acl>
          <resources>
            <all>
              <title>Allow Everything</title>
            </all>
            <admin>
             <children>
                <customer translate="title" module="seller">
                  <children>
                    <set_time translate="title">
                    <title>Seller List</title>
                    </set_time>
                  </children>
                </customer>
              </children>
            </admin>
          </resources>
        </acl>
    </config>

Accoring to you have create new menu at customer section a child tab and it name is set_time So i am add this code

    <customer translate="title" module="seller">
      <children>
        <set_time translate="title">
        <title>Seller List</title>
        </set_time>
      </children>
    </customer>