If you remember the ports that you had forwarded for eMule, go to preferences>connection>connection and set the same ports for eMule Plus. Restart eMule Plus after doing this.
Or you could see which ports eMule Plus is using, and do port forwarding on those ports. Whichever you find easier. http://www.portforward.com
has detailed instructions for a large number of routers/modems.
You will also need to delete any previous rules for emule.exe and when the firewall asks you again, allow fresh permission to emule.exe to both connect to the internet and accept connections from the internet.