Since Linksys (or the firmware vendor which might be gemtek or cybertan) disabled the networking within the redboot bootloader (and also the partition management), we have to replace it as first step
to get it working. this is only possible with serial console right now

setup your local lan network to 192.168.1.72 and run a local tftp server (see provided tftp server for windows)
then enter the following line into the redboot with serial console (baudrate 9600)

ip_address -l 192.168.1.1 -h 192.168.1.72
this will enable network access

now do 
load -r -b 0x80041000 redboot_ap65.ram
exec

a new redboot will now start

now do the following commands:

fis init 
load -r -b 0x80080000 redboot_ap65.rom
fis create -l 0x10000 -e 0xbfc00000 RedBoot
reset

the new bootloader starts now

now do

fis init
load -r -b 0x80041000 linux.bin
fis create linux

and config the bootscript with fconfig
the script must be
fis load -l linux
exec

save the config and type "reset" again to boot up the system

to enter the redboot console with this new bootloader you must push the reset button. this will also allow to enter the tftp recovery 
mode which allows to send a dd-wrt webflash file with tftp to the bootloader for flashing.