HI,
Does the Printer have a USB Option ?
ON a LAN a router would most likely issue IP addresses to the computers and Ethernet connected devices.
The Add Printer using an IP address is then looking for similar LAN IP numbers to the one the computer is getting.
When the computer Shares it's Internet Connection it acts as a router and gives the connected device(s) their own IP numbers.
These are unlikely to be in the range the computer is getting from any router or Modem.
I have not done this but does the System Preferences > Sharing > Internet Sharing give you any clues to the IP being used ?
It is most likely in what is called the self assigned range that starts 169.xxx.xxx.xxx
8:46 pm Sunday; August 11, 2013
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.4)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad
"Limit the Logs to the Bits above Binary Images." No, Seriously