VirtualBox: How to share a guest's (XP) internet with the host (OS X)

I have an internet connection that can only be used on Windows operating systems (our school requires that we install an .exe login client to connect to the dorm internet). As I am currently using a Mac, my goal is to get the internet running in XP in a virtual machine, and then share the virtual machine's internet with the host. Here's what I've done so far (I'm pretty new to all this, so sorry if I leave out anything important!):

1) Install VirtualBox, create a new XP virtual machine
2) Add a Bridged Adaptor NIC to the guest allowing it access to the ethernet port

With this, I can use the internet fine in the guest, but have no internet in the host. (Note: Making the NIC NAT does not work. When set to NAT, the login client in XP can't find the school's server and I can't use the internet). The next step is getting the guest to share it's internet with the host. Here's what I've tried:

3) Add a second Host-only Adaptor NIC to the guest specifying the following:

IP: 192.168.22.2
Network Mask: 225.225.225.0
DHCP Server unclicked (disabled)

4) In the guest, enable Internet Connection Sharing for the internet connection.

At this point, LAN 2 in the guest (the connection to the host, I think) gets automatically configured with an IP of 192.168.22.1 and the same mask as above. Back in the host, I can ping 192.168.22.1 and get a response. But no internet.

I feel like I'm missing a step, namely how do I instruct the OS X host to use the Host-only connection as its internet connection? I can see the connection ("vboxnet0") when I use ifconfig in the terminal but I have no idea how to use the connection for internet. Anyone know how I can do this?

Thanks for your help!