Hi.
I couldn't find the option to switch on Call Waiting on my 8800 when I first got it, so I just used the GSM codes:
Enter them into the phone application and hit "Send"
Interrogate Call Waiting Current Status: *#43#
Activate Call Waiting: *43#
Deactivate Call Waiting: #43#
You'll get an answer from the network informing you that the changes have taken place.
Hope that helps