@livetorock, as mvc said it should be automatically assigned.
If not, and if I were you I would first try to reset that router you are trying to use as an access point.
If it's a new router, I'll skip the reset part.
I'm assuming this scenario here, lets call the router by letters A and B:
-Main WiFi router(A) connected through the ethernet cable to your ADSL modem --> and DSL line connected to your ADSL modem.
-Second WiFi router(B), you are going to use as an access point.
Here's what you do:
1. Hook up your router A( I assume it's already hooked up), I know it's the main one and make sure Internet is working.
2. I'm assuming you are using windows , so start run command and type 'cmd' command.
3. Type in 'ipconfig/all' without quotes.
4. Now take a note of DNS server IP. That's usually your routers gateway IP.
5. Disconnect your router A from your ADSL modem and connect router B
6. Router B should take everything automatically but you don't want it to do automatically.
7. Take a note of it's gateway IP of your router B(it should be same as of main router A but sometimes it differs depending on make and model of the router), type it in the web browser(it should show the router's config page) and manually change the dns server to the one you took a note @ step 4( update the dns server with the gateway IP you got from router A). I think depending on the router model, you need to find that page on the router's config page and manually update the primary dns server. Also give access point name(for eg: MyApoint) of your choice, take a note of it so that devices in proximity could easily find it. You can 'wep/wpa' protect it or not, it's upto you. Save it, click Ok.
8. Disconnect your router B from your ADSL modem and connect back Router A to it. Make sure internet is working.
9. Now, that your Router B is setup, all you need is power adapter to the Router B and hook it up anywhere there is power outlet.
10. It should be up and running and basically any devices near to your Router B should easily connect to the internet.
Alsmo make sure you don't go too far else it's not gonna work. Walls and obstacles in between the Main router and access router do matter.
I usually do this to extend any wi-fi router's range. This is an old method but there are another devices specially made for cases like this called repeater, it's easier than this.
This is just a basic idea and it's not complicated. Hope it helps you.