Could you advise which Editor you are using please?
When you include the item is it in a Paragraph <P tag? If Yes try selecting the <div tag setting in the editor
When you write
I can not insert a Jdownload file into the EventBooking editor (From Joomdonation).
are you using the jD Button plugin?
Also slightly confused by you reference to JComments. Is that a separate problem?
To check further could you please send me a Private Message with your site address. May need super user access later. Also please describe how I get to the problem(s)
PS to send a PM click on the whitish 'square' below my name