This article is an excerpt from the Installation Guide: Nintex Workflow 2013 and Nintex Forms 2013
Nintex icons are missing from Central Administration.
This indicates that for some reason the Central Administration Feature was not activated. It's possible that the Nintex Workflow or Nintex Forms solution was deployed to the front-end web servers but not to the server hosting SharePoint Central Administration.
To resolve this issue, deploy the Nintex Workflow and Nintex Forms solutions to the Central Administration Web application; if needed, activate the relevant Nintex features manually using Stsadm commands.
Run the following command, where http://centraladminurl:port is replaced with the URL and port for the Central Administration Web application.
stsadm.exe -o activatefeature -name nintexworkflowadmin -url http://centraladminurl:port
Run the following command, where http://centraladminurl:port is replaced with the URL and port for the Central Administration Web application.
stsadm.exe -o activatefeature -name NintexFormsAdminWeb -url http://centraladminurl:port