In order to use TopStyle with InterDev, you must configure TopStyle as an external browser. To do this, select and add TopStyle to the list of browsers.
Once this is done, you can right-click on a style sheet in an InterDev project and select to edit it in TopStyle. You can also select from the "View" menu when editing a CSS document in InterDev.
Note: This process does not work when TopStyle has been added to the list of "Open With" applications, which is why you need to add TopStyle to the "Browse With" list instead.