You don't search on ebay. Go to http://www.live.com/ and search for something that is likely to be on ebay. I know Nikon D80 works since that is what I bought this morning, but any other digital camera, I-Pod, Zune, etc. should work. I just searched again now, and it still shows up as 30%. In the results page, there should be a gray "sponsored sites" box, and one of the sponsored sites should be ebay with the $ Cashback sign and some text with the current percentage. Click on the ebay link, and you will be directed to the ebay results page.

Once there, you can search for other things. Just remember like kirax2 mentioned that the cashback applies to Buy it Now and using PayPal to pay. While shopping and checking out, you should be able to see the Microsoft Cashback icon on your screen. Afterwards, you will receive a message from Ebay in you My Messages saying that you qualify for Microsoft Cashback and then just follow the instructions from there.

I hope that helped!