Easy when you know how! I never had to do this before but I found the answer online:
First, connect your PIXMA TS5120 to your wireless network:
1. Press the HOME button on your PIXMA TS5120.
2. Use the arrow keys to select Setup on the Home screen and press OK.
3. Use the arrow keys to select Settings and press OK.
4. Use the arrow keys to select Device Settings and press OK.
5. Use the arrow keys to select LAN settings and press OK.
6. Select Wireless LAN and press OK.
7. Use the arrow keys to select Wireless LAN setup and press OK.
8. Use the arrow keys to select Manual connect and press OK.
9. Select your wireless network from the list that appears and press OK.
10. Press OK. Once the keyboard appears, use the arrow keys to enter your Wi-Fi password and press OK on each letter/number.
11. Once entered, use the arrow keys to select OK in the lower right hand corner and press the OK button.
12. At the Enter passphrase screen, use the down arrow key to select OK and press the OK button.
13. Press OK at the Connected to the wireless router screen.
14. Press the HOME button.
Your PIXMA TS5120 is now connected to your wireless network.