custom recurrence for appointments in evolution

[t011]

I'm using Ximian Evolution 1.4.3 and I'm trying to create a recurring appointment. I see that you can set the recurrence for "no", "simple", and "custom". But the custom recurrence radio control is ghosted so that I can't use it. Is there a way to enable this feature? Or is this a feature that doesn't exist yet in Evolution? What options are available with the custom feature? I'm trying to create a recurring appointment that happens on the third Friday of every month. Thanks in advance.