I have an Asus RT-N65R wireless router, When setting up the 3g USB modem, it asks for the ISP. The ISP isn't listed.
Don't worry, this is a very common situation and your ISP has given you all the details you need. Please attach the WAN cable and see if it configured itself automatically. If the third LED from the top lights up, the Internet connection should work.
If not, search in your ISP's documents for one of the following terms: PPPoE, PPTP or L2TP. These are protocolls for the modem to set up an Internet connection.
Your router's manual can be found online: http://usa.asus.com/Networks/Wireless_Routers/RTN65R/#download