If you installed an elgg site the first person to register is the Admin. Once you login you should see an additional link in the top toolbar , to the right of the "settings" link. The link says "administrator".

 Hope this helps.  

Do you mean that you forgot your login? if so click lost password link and have it emailed

I first install it, but cannot create new user, I don't why

Alert is :  the requested action(register) was not defined in the system.

@badjohnny - Checkout .htaccess

Check the rewrite rules in the .htaccess. I think there is something wrong over there.

I don't know where I should modify？