@Petedd From what I can tell, reading the DD-WRT documentation and forums, it depends on whether your router is Broadcom or Atheros chip based.
Broadcom chips use one filter list and all virtual SSIDs beneath that inherit the filters. Atheros chips can use multiple filter lists.
This means if you have a Broadcom chip (found in most routers) you'll have to add the guests MAC addresses to the main filter list in order to allow them access to the guest network. That's counter productive to the whole idea of having a guest network. I'd suggest ditching the MAC filter altogether. As long as you're using strong encryption like WPA-2 the MAC filtering adds little security to the whole setup (and in this case, a lot of inconvenience).