10-14-2011, 08:42 PM
Ok, so here's how I understand you are set up.
You use a 3G modem which is plugged into your iMac (I'm assuming this is like Clear or some other device using a cellular signal to provide an Internet connection), then you share that connection out to your iPad through the Airport in your iMac.
Assuming I've got that correct, what you are wanting to do is not possible.
Your iMac is not creating a wireless network, at least not in the sense that you would need for the Time Capsule to work. What you are creating is called an ad-hoc connection, which is a strictly peer-to-peer connection, that is, it only operates between two points (in your case, the iMac and iPad). You may be able to connect other devices to that connection simultaneously and access the Internet, but they cannot see each other on the network, because the infrastructure is not there.
Now, not all hope is lost. There are devices you can purchase which accept a USB 3G modem like the one you have and can provide a wired or wireless network connection as well. Here is a link to an example: http://3gstore.com/product/3027_cbr4...fi-router.html
What you do is plug your USB 3G modem into that device, then plug an ethernet cable from the WAN port on your Time Capsule to the LAN port on that device. Some configuration may be necessary, but the end result is that your Time Capsule will create a proper network and use the 3G modem for its Internet connection.