Apple have not updated their documentation for airports for a long time.. and now they have dropped the line altogether.. perhaps that is the sign of impending doom.
This is easy peasy though so we can give you simple instructions.
1. Reset the TC to factory. No files are lost doing this if you want to keep backups etc on the hard disk.
(Factory reset you hold in the reset button before you power on.. and keep holding for about 10sec after power on until front LED flashes rapidly.)
2. Place the TC near the airport extreme and power it up. No other connections.
3. Open your airport utility in the computer and click Other Wi-Fi Devices (1) which shows a new item is available and not configured.
4. The setup wizard will begin.
This will pretty much do everything for you.
Give a suitable name to your new base station.. NOT APPLE RECOMMENDED NAME.
Keep it short, no spaces and pure alphanumerics.
5. From that point it is pretty well automagical.
Your TC will get wireless name and password as well as main password from the Extreme.
6. Once completed you should be able to check everything is good in the airport utility.
Note a few things here.
1. Wireless links are dotted.. ethernet links are solid lines.
There is a lot of sense in that as using wired links is far superior to using wireless extend. Very dotty.
2. Where it says connection: Unknown is important info as to how well the link is working. If you hover your mouse over the word Unknown you will find it is known very accurately. That will give you the data rate, RSSI and band being used for the connection. It is really important for the TC to be close enough to the extreme .. especially your ancient Gen2 to actually get decent signal.. otherwise it cannot extend with any speed or reliability.
I would want RSSI of no less than -60dBm.
I would strongly recommend you buy a Gen5 Extreme to replace the Gen2. You can find them on ebay for $20-40.. and it will pair very well with the Gen4 TC as they are same release models.
