If you go to Tools -> Internet Options, Advanced tab, and scroll to the bottom, you'll find an option to "Warn about certificate address mismatch", which you can disable; the change will take effect after you restart IE, and should stop the browser from complaining about the cert.
Unfortunately, it'll also stop the browser from complaining about address mismatches on every other site you visit, as well. That's less than ideal, but it's the kind of tradeoff you get to make when you use IE.
Of course, installing a cert with the right address on it (or, if it's a subdomain you're visiting and the cert's issued for a higher-level domain, a wildcard cert) would be the right answer, but if you can't do that, this will at least suppress the warning.
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