[maij]

you are trying to do format and showplayerdialog in one line.
In pawn, u can't do that.

format(stringsomething,lengthofmessage,"Welcome to the server\n\nRegister Name: %s\nPlease open a new account\n\nEnter a password:",plname);

ShowPlayerDialog(playerid,50,DIALOG_STYLE_INPUT,"R egister",stringsomething,"Register","Quit");

I've been out pawn for a long time, this might not be correct but you get the idea.