MD5 is a standard algorithm that hasn't changed since 1991, and it's been supported by Java since version 1.0. What do you mean by "not compatible with Java 7"? Also, what is "HTTPClient.MD5"? Can't you just use the implementation that's built into the JRE class libraries instead?
I guess it's probably from this library. The specific class can be found here. It's an undocumented package-private class project last updated in 2001, with a very poorly-named package that causes unnecessary confusion. I would think the original poster would be much better off with something of a more recent vintage, something actively maintained and used by people. Like the library he linked to, but isn't actually using.