If you connect Fritz!Box the first time to your Internet Provider, but Fritz!Box can't connect to the ADSL Signal even after 10 minutes (TheDSL Light keeps blinking)
make sure Username and Password are correct
check the VPI/VCI settings and Encapsulation (Menu: Internet / Account Information)
Ask your DSL Provider for the values required. Please note that manual settings are required only in special cases.
Often you can also find the values by simply searching