Steps to add custom liferay portlet into control panel:

To add custom portlet into control panel need to write the following property 

1. liferay-portlet.xml file

<control-panel-entry-category>users</control-panel-entry-category>
<control-panel-entry-weight>1.0</control-panel-entry-weight>

For ref: Please find below the code of liferay-portlet.xml fil

<?xml version="1.0"?>
<!DOCTYPE liferay-portlet-app PUBLIC "-//Liferay//DTD Portlet Application 6.1.0//EN" "http://www.liferay.com/dtd/liferay-portlet-app_6_1_0.dtd">

<liferay-portlet-app> 
 <portlet>
  <portlet-name>newportlet</portlet-name>
  <icon>/icon.png</icon>
  <control-panel-entry-category>users</control-panel-entry-category>
  <control-panel-entry-weight>1.0</control-panel-entry-weight>
  <instanceable>false</instanceable>  
  <header-portlet-css>/css/main.css</header-portlet-css>
  <footer-portlet-javascript>
   /js/main.js
  </footer-portlet-javascript>
  <css-class-wrapper>newportlet-portlet</css-class-wrapper>    
 </portlet>
 <role-mapper>
  <role-name>administrator</role-name>
  <role-link>Administrator</role-link>
 </role-mapper>
 <role-mapper>
  <role-name>guest</role-name>
  <role-link>Guest</role-link>
 </role-mapper>
 <role-mapper>
  <role-name>power-user</role-name>
  <role-link>Power User</role-link>
 </role-mapper>
 <role-mapper>
  <role-name>user</role-name>
  <role-link>User</role-link>
 </role-mapper>
</liferay-portlet-app>

2. liferay-display.xml file

user category.hidden into name attribute of category tag

<?xml version="1.0"?>
<!DOCTYPE display PUBLIC "-//Liferay//DTD Display 6.1.0//EN" "http://www.liferay.com/dtd/liferay-display_6_1_0.dtd">

<display>
 <category name="category.hidden">
  <portlet id="newportlet"></portlet>
 </category>
</display>