First off, you should connect the computer directly to the router via an Ethernet cable to attempt this configuration.
In order to use your router's web portal, you'll need its IP address. This can vary depending on how the router was setup, but it should be easy enough to find. On one of the connected computers, you'll need to look up the "default gateway". Assuming your housemates are using Windows, it can be done by doing the following:
Start => Run => Type:
CMD
..press Enter.
This should bring up a command prompt. In the command prompt, type:
...press Enter.
One of the tidbits should be the default gateway:
(The window pictured above is from a VM session - your address should be 192.168.1.1 or something similar).
If you're doing it on your Mac, you can see your default gateway by going to the Terminal (Applications => Utilities => Terminal) and type:
Code:
netstat -nr | grep '^default'
...which should yield something like:
Then, on one of the connected computers, open a web browser and type that default gateway address into the address bar in the browser. You'll be prompted for a username/password, these should have been setup during the installation process. If you didn't set them up, or left them as default, the username will be admin and the password will be blank. Hit Enter and you should be taken to the portal page.
Security settings are configured in Setup tab => Wireless Settings (left window pane) => Manual Wireless Settings (middle pane) => Wireless Security mode section:
Here, you'll choose the settings as shown above. Be sure to set a "Pre Shared Key" and save your settings. The "Pre Shared Key" is the password you'll be prompted to enter on each one of your client devices.
I know it sounds like a lot, but not only will your network be more secure, it should be faster and a lot more reliable.