Nice to know that you have crossed some initial hurdles....
1. you have configured your modem in PPPoE
2. you have got your DIR-300 to transmit wifi signals....
Always configure your DIR-300 while connected to it through a RJ-45 cable (do not attempt to configure in wifi mode because you will lose connectivity during router reboot)...
You may leave the Enable box "unchecked" in WiFi protected setup and use custom settings in "Wireless Notwork Settings" and "Wireless Security Mode"...
Decide which "security mode" you would like to implement.... start with "Disable wireless security", if you are able to connect and surf through wifi, then you may go to WPA/WPA2 mode....
1. you have configured your modem in PPPoE
2. you have got your DIR-300 to transmit wifi signals....
Always configure your DIR-300 while connected to it through a RJ-45 cable (do not attempt to configure in wifi mode because you will lose connectivity during router reboot)...
You may leave the Enable box "unchecked" in WiFi protected setup and use custom settings in "Wireless Notwork Settings" and "Wireless Security Mode"...
Decide which "security mode" you would like to implement.... start with "Disable wireless security", if you are able to connect and surf through wifi, then you may go to WPA/WPA2 mode....