Follow thess instructions.
Send an SMS to
49001 writing: "TIM 12345678" (without "")
Where 12345678 is the password you decide (of course 12345678 is not accepted, it can be alphanumeric).
At this point you should receive 2 SMS: one at once and one a bit later, about 1 hour.
Go here:
https://www.tim.it/acquista/servizi/...harge_amount=5
And use your mobile number (without +39, I guess) as login and the password you chose before as password. Click con "entra" and you are in, refilling 5 euros on the number you sent the SMS from.