Yes, the plugin is supposed to allow you to add content using the regular methods. Basically, this just creates a new post type, so adding images, files, text, videos, etc. can be inserted into the post content area (just lik you could with any other post).
There is no AJAX window to add a new instruction - it takes you to the instructions section, and there's a "Write Instruction" area - exactly the same as the "write post" area. Do you mean you can't upload an image when you click the "Add Image"icon? (That shouldn't be affected, either.)
I don't know if the Greek characters are causing the issue - but it's a possibility.
Have you tried deactivating your theme and your plugin (and reactivating one at a time) to see if something else is conflicting?