This isn't a trivial thing to do until the IronRuby Visual Studio Integration component is out. So here are the steps in order to achieve that:
1. In Visual Studio, click on File-> Open -> Project/Solution
2. Select ir.exe from the [IronRuby code directory]\trunk\build\debug (or release, depends on how you've compiled the code)
3. Right click ir.exe in Solution Explorer and select Properties
4. In Command Arguments, with "-D [path to code file]" where [path to code file] will be the full path to your ruby file.
For example, -D "c:\dev\IronRubyTest\test.rb".
That's it! you can debug your IronRuby code file now!
This post was greatly inspired by the IronPython solution that was posted by Harry Pierson on his blog.