is yr NAT open?

you can try setting yr xbox to a static IP, then use DMZ on router to point to that IP, if it doesnt work you try putting the Static IP of your imac/pc.

this is a temp solution to test out

hmm if im not wrong ifconfig en0 = ipconfig on windows, which en0 is only relate to yr first Network card IP, i have no idea how to setup ICS on MacOSX but you should use a same range IP not the one your currently using?

if your network card on MacOSX is 192.168.2.1 with gateway 192.168.2.2, your Xbox should be 192.168.2.* (somethings other than 1) , gateway should be same 192.168.2.2

as i have said you need to setup yr router DMZ enable it & try with this 2 IP 192.168.2.1 or 192.168.2.* (yr xbox IP), see which would let you get an OPEN NAT Type. remember to reboot yr router & xbox after you have set the DMZ settings to see the result.

@creepycreeper you cannot have same IP address on the same network, if your mac is 192.168.2.1 then yr xbox should be an example of 192.168.2.2. the reason i asked you to try yr DMZ with 2 IP above its bcoz i have no idea which IP you should use, sometimes to see the OPEN NAT result you might have to reboot the router to see the effect.