go to ebay and save the file to your computer (can click the link and then save, or right click and save target)
once you have that saved, you will have to convert it into a file that googlebase can read.
there's a couple of converters out there. the one i used is http://www.gamegiants.net/gbase
that page will walk you through converting the file.
once you have the converted file saved to your computer you go back to googlebase http://base.google.com/base/dashboard and select item type as "Product" from the dropdown list and upload your file
You will also want to set your profile info at http://base.google.com/base/settings use keywords in description and put your ebay store url in the website url box.
NOTE: you can add more keywords and other info to your file before uploading it. simply edit it in excel and add them in, then save as a tab deliminated file. IMPORTANT: There are some columns you will want to leave blank in the file (such as the id column. you can put that in there, but you have to be very careful! see alphabetical list of attributes and their meanings for specifics for each column)
instructions on the file is here http://base.google.com/base/tab_instructions.html and an alphabetical list of attributes and their meanings is at http://base.google.com/base/tab_attributes.html
Dont have Excel? then download openoffice link in siggy