MacDrive works with Windows running virtually under Mac OS X using Parallels Desktop for Mac or VMware Fusion, with the exception in both cases that the Mac OS X system volume will be inaccessible.
In such cases, the Mac OS X system volume is inaccessible because, as with any operating system, Mac OS X requires exclusive access to any volumes it has mounted.
Please note that you will need to configure your virtualization software to have a particular virtual OS take control of a drive before Windows can use it. This is often in a "Settings" or "Devices" menu. For detailed help with your particular virtualization product and version, please contact the software manufacturer.