I first wrote here: https://premium.wpmudev.org/forums/topic/theres-no-way-to-actually-add-content-to-the-newsletter
... but the ticket is marked as solved... while the problem is not!
So, same problem here: incompatibility between fusion-core and enewsletter
You are aware of this problem since may 2016, and there is no update ?
No one thought about the possibility of creating a newsletter on another WP installation (without core fusion) and then exporting the newsletters and finally importing them into the WP where fusion core is installed ?
I find a bit odd that newsletters can not be exported / imported ... unless I have not seen all the options, and I would glad you to showed me how to do it!