On DD-WRT Router, go to Services >> Zabbix Agent and input Zabbix Server IP >> Save >> Apply Settings.
On the Server, add the host as per usual, but first you nee to import the following template and attach it to the host.
http://svn.dd-wrt.com/export/31647/src/router/zabbix/config/zbx_template.xml
http://svn.dd-wrt.com/browser/src/router/zabbix/config/zbx_template.xml