I'm moving your question to the InDesign EPUB forum.
You need to set up an EPUB reader for your EPUB file.
First, make sure that you have installed either iBook (if you're on a Mac) or Adobe Digital Editions (Mac or Windows). The latter is a free reader which you can download from www.adobe.com.
If you're exporting a reflowable EPUB (File > Export > EPUB (Reflowable) or a Fixed Layout EPUB (File > Export > EPUB (Fixed Layout), on the Viewing Apps panel, select the reader you want to open the EPUB.
If you're using an earlier version of InDesign, you may not have that option. Make sure you have a reader installed, and use your operating system method to associate an EPUB file with that reader.
You as well as ePub readers such as the Kobo, you should be able to view an ePub on the Adobe Digital Editions ePub viewer, which you can download here:
If you have a Mac the iBooks viewer is even better:
And there's Readium, a plug-in for the Chrome browser: http://readium.org
If these don't work, you probably have something wrong with your ePub – try and validate it using this:
Thanks, I got it.