The key point here is that when you click the Telnet link, it's just providing a URL like this: telnet://X.X.X.X to your browser. IMC doesn't tell the client which Telnet program to use - it's up to your browser to interpret this.
Different browsers have different ways of handling it. Internet Explorer makes it difficult to enable this, but it's do-able. Once enabled in IE, you can use the registry to change the program that gets called.
With Firefox, the first time it opens a telnet:// link, it will ask what program you want to use. You can set it to always use that program.
Note that this is different to the SSH link in iMC, which uses a Java applet to launch a program. That is configurable within iMC, as opposed to the browser.
But of course, you're using SSH everywhere, right? Right?
#ssh