We're sorry for the trouble you're experiencing.
The current CC 2017 plug-in is a 'Beta' version and some features or functions may not work as intended. One of those being 'Multi Device Export', as this function does not currently work in the 'Mag+ Designd Vertical Panel' yet.
As a workaround, you can use 'Folder Multi Device Export' (more on this here: Mag+ Menu - Using the Folder Multi-Device Export) from the 'Mag+ Designd' pulldown options (within InDesign) in the meantime.
We're hoping to have a fully functional CC 2017 plug-in out soon. In addition, if you haven't already, we encourage you to bookmark our Release Notes forum and Announcements forum this way you can stay on top of the latest from us including announcements, new features, latest bug fixes, etc.