If you didn't setup username/password before, then they are admin/admin. Otherwise, you need to reset the router to factory default. Then you can use admin/admin to access your router.
Dell™ TrueMobile™ 2300 Wireless Broadband Router User's Guide
http://support.dell.com/support/edocs/network/p57205/en/config/index.htm
http://support.dell.com/support/edocs/network/p57205/en/config/web_conf/index.htm#overview
