Sqlmap Help.. KaliLinux

sqlmap -u http://metin2lf.ro/index.php?page=register --dbms MYSQL --level 5 --risk 3

Is there something wrong with my command? It says referrer parameter "referrer" is not injectable. I raised level and risk gradually from 1 to maxim. But now what's next...? I think that MYSQL is right, I had a server like this about 2-3 years ago. Why I can't use sqlmap? Do I need a numeric parameter for eg php?id=1?. Is the server overprotected? it's powered by nginx...

It's just an online game. Some friends hacked some databases of another servers ( everyone can have a server of this game, you need just a host ) but they tried manually by guessing accounts. I don't wanna to destroy that server or cause any damage, i'm friend with the owner, but I just want to know.. if sqlmap can do it, and what i'm doing wrong.

Sorry for my "noobism" :D.

P.S: Sorry for my... bad English :(.