Yahoo backlink checker tools or sitexplorer tools is shut down last year. Now yahoo doesn't have such for checking backlinks so ignore them.
SEO Spyglass is a good alternative, it has many search engines working together to find all your backlinks
Yahoo Site explorer is redirect on Bing webmaster tool so just go to your bing webmaster tool and check your back links details.
The best backlink checker tool is Google webmaster. The following is some information on Yahoo backlinks: Backlinks from Yahoo! Directory can be valuable due to the authority of the domain, age of the directory, and general quality of the directory. There is a substantial fee to have a website reviewed for placement, and this does not guarantee that a website will be included in the directory. A backlink from Yahoo! Directory isn't necessarily going to boost your rankings in search engines, but it can help increase exposure and combined with good SEO can help nudge a website towards its goals. Useful for researching other websites in your market, or checking up on your own links, the Yahoo! Directory Backlink Checker will quickly tell you how many links any domain has in this authority directory.
pagerank.net is the tool which i use to check the pagerank of my website/pages and also Google, Yahoo backlinks.