Moving to InDesign EPUB forum.
Honestly, while you might be able to produce EPUB from CS5, you'll have to go through a ton of work to do so. You really need a modern version of InDesign CC to be productive with EPUB export.
I can't help you with your export problem because I'm not a Windows user.
Steve’s right, even if it worked, it wouldn’t work.
CS5’s EPUB tools were archaic at best. Additionally, you haven’t told us what operating system you’re running. CS5 is not supported under the latest Windows and Mac operating systems.
He wrote Windows 7.
Yeah, I missed that, but the point still stands.
CS5 for EPUB is insane.