You'll need to give your computer a static IP address on the same range as the DD-WRT router to be able to access it.

This can be done on either the ethernet or WiFi interface. Once the static IP address has been set to an IP address in the same range ( should start with 192.168.1.2 and end with 192.168.1.254 ), you'll be able to point your computer to 192.168.1.1 and access the DD-WRT configuration page.

As I am sure you have found, DD-WRT documentation is lacking vital information in some areas. beware that on some models repeater bridge makes the router unstable, requiring frequent reboots. sometimes its just the hardware and sometimes it can be resolved with a different build# of DD-WRT found in the dev section. (try to always run stable, and none-beta versions) this limitation is non-existent on routers with two physical radios.

Set the primary router DHCP range to "192.168.1.2 through 192.168.2.249"

This is how to configure it on a router with one radio.

Plug directly into the second router while configuring it. (this is assuming you have factory reset it) In the second routers configuration page go to the setup tab.

dd-wrt setup tab

Put the second router at the end of the first routers sub-net. -"192.168.1.254 /24"
Put it in DHCP forwarding mode and point it to the main router IP. -"192.168.1.1"
WAN & STP disabled.
Now would be a good time to enter a NTP server. example: "pool.ntp.org".
(this will give the router an accurate system time)
Hit Save.

In the Wireless tab --> Basic settings

wireless setup tab

Set the Wireless mode to Client Bridge.
Set the wireless network mode to what wireless mode you want it to connect to the first.
(choose the fastest mode both support).
Set your first routers SSID into the wireless network name box, or you can automatically populate it under Status -> Wireless. Hit Save, then go to the Wireless Security sub-tab and make sure your pre-shared key is correct. do not create the virtual interface just yet.
Hit Save.

In the Security Tab

Disable the SPI firewall.
Hit Save.

Reboot the router under the administration tab. Plugged in wired to the router, it will come back up with the .254 address, you should be able to go to Status -> Wireless and see that you are connected to the first routers wireless network. (with noise readings, RSSI etc) You should be given a DHCP IP address from inside the first routers Subnet.

go back to the wireless tab and create the virtual WLAN interface.

wireless setup tab

Hit Save.
don't forget to set the WPA-PSK for the virtual interface under the wireless security tab.

this brings up a choice, you can make this a separate SSID from the first with a different password, or you can match the settings from the first exactly, and the clients will automatically roam.

At this point i would recommend that you either setup the connection watchdog, or a timed reboot under the administration tab. (i find that the timed reboot works well for 24 hour periods)

Administration tab -> keep-alive

once set, hit save, then apply. and finally reboot the router. backup these settings to a config file under the "admin --> backup tab". if this doesn't work for you, try another build release. use the backup you created to save yourself time if you do need to try another version

Hope this helps you out. Let me know if you run into trouble.