Here are some new facts that might help you get your bearings:
It would be wise to get Mac OS 7.5.5 (a free upgrade) or later because it fixes a lot of bugs in 7.5 and, more importantly, it includes Open Transport, Apple's much improved networking interface. Open Transport makes TCP/IP much more reliable than under Mac OS 7.5.
The reason that your hub doesn't see the Mac may simply be that the software needs to be set up first.
Your Mac's Ethernet port is probably not active by default. It is only turned on when you activate it by switching AppleTalk or MacTCP to use Ethernet. Which control panel were you in when you got the error message, and what was the exact text of the message?
I'm not sure I remember which control panels are in Mac OS 7.5 vs. 7.5.5. If you have an AppleTalk control panel, choose Ethernet from the "Connect via:" drop-down.
You shouldn't need to change anything in the Chooser (which you'll see in the Apple menu).
Once the software is set up, you should be able to connect to a regular hub port. If switching AppleTalk to Ethernet complains that it can't configure the port, then the Mac is not seeing an Ethernet network, so try the uplink port instead. If both fail, I suspect that you need Mac OS 7.5.5 or later.
Regarding software...
DAVE works well on the Mac, but getting a version that works on your system will be difficult. The current shipping version (4.0) only does Mac OS 8.6 and up. I don't know what version you would need. You could contact
Thursby Software to see if they can supply an older version.
PC MacLAN would probably make more sense. The current version works well for Windows XP and will let it talk to any Mac using AppleTalk. It must be the current version though to work on XP.
By the way, do you have a CD-ROM drive?
Since you had to wait until you got home and then posted at 3pm Pacific time, I assume you are on the east side of the U.S. (I'm on the west). Otherwise, I'd drive right over there with my Mac OS 7.6.1 CD-ROM and fix it myself!! Of course, then my dinner would get cold...