You should be able to assign mac to a group and then apply the rules to that group just using the gui. Worse case I know you can do it with the iptables command.
man i don't really want to do all that command typing have you ever heard of anybody else having the same problem where they set everything up in the router gui and it still don't work. Which brings me to another question is it possible for the dd-wrt to still load on a router but could have installed somewhat not right and have some flaws? finally is there a software i could use with dd-wrt that could solve my problems with account restrictions?