You only need to fill in this form if you are applying to use the wired (Ethernet) connection. You do not need to fill it in to use wifi.
The wifi connection is over Eduroam. To use it you need to follow these instructions, which involves getting a token and for some users involves installing a small tool. Make sure you get these before you come to Cambridge if you can as you will get online a lot quicker.
Use the IT ticket system and select Wired Network Application from the help topics: http://itsupport.caths.cam.ac.uk/