Hi,
We could use a little more information...
If you can connect to the Macintosh and the Mac is able to connect to the internet, why can't the Indy running 5.3 connect to the internet?
If you need more information on how to, we would be happy to respond. Basically, point the Indy at the same gateway your mac uses. Check out how in the man pages:
man resolv.conf
man route - look at "route add default"
Reply if you need more information. Good luck.