Receive Wi-Fi via dongle, redistribute via Airport
It sounds like you're trying to NAT behind your old Macbook. I assume you're using Internet Sharing (System Preferences -> Sharing) to do so, which will auto-configure a NAT for you.

If you are able to connect to the Macbook's shared connection at all you should check the routing table of the machine you are connecting to the Macbook NAT from. You might have a duplicate route, or traffic might be going the wrong way.

You can do this via Network Utility (GUI for netstat). In Network Utility on your client machine go to the Netstat tab, click the radio button "Display routing table information", and then press "Netstat".

If you run an ifconfig on your Macbook, the IP of a secondary interface (br0, tun0, etc OR your internal card - NOT your wifi dongle) should be a local IP - i.e. 192.168.x.x or 10.0.x.x. Ensure that the default gateway in your client machine's routing table is the same as the IP of the network interface that corresponds to your shared connection.

You can change the default gateway via the "Network" System Preferences pane, or using the route command, i.e.

route delete default
route add default &lt;Macbook ip&gt;