How do I open inDesign CS5 files?

Solution 1:

The development version of Scribus can open IDML files (InDesign Markup Language, the official import/export file format for applications that have to work together with InDesign). I am not sure how good the IDML support is currently, so you will have to test yourself...

If you want to try the latest development version of Scribus, there is a PPA for that. But remember that these are daily builds of the development version, and they are probably not ready to be used for production work yet (and every day a new version will be available, which may fix some issues, but also introduce new ones).

Solution 2:

I think you can export a PDF file in inDesign and then import it in Inkscape.