StopSpazzing is onto something.
...I changed everything in BIOS, computer first look for USB-ZIP drive...
Maybe you should try USB-Floppy or USB-HDD or other USB options? (There are also USB Legacy options in the BIOS, for example).
...After unzipping ubcd503.iso...
title UBCD 5
find --set-root /ubcd50rc2.iso
map /ubcd50rc2.iso (hd32)
That menu won't find ubcd50rc2.iso, for 2 reasons. First, the iso image name you downloaded is for version 503: ubcd503.iso. Second, either you
the iso image as you said, and used the method described in the wiki to put ubcd in your USB, so ubcd503.iso is not
there in the usb, or;
B) the iso image shouldn't be unzipped and you used the alternative method of RM's tool, with the complete iso in the usb.
So, you have to follow the instructions correctly (choose one method only; that's why I suggested to follow the script instructions), and you need to configure the BIOS correctly.