I am sorry to hear about your problem. I have been making dailies / etc. with REDCINE-X, Avid MC 5.x 9 weeks this year more or less exactly like the other gentlemen described: export MXF / AAF DNxHD36 / 1080p from redcine-x (pro), move the created MXF to the AVID MEDIAFILES/MXF/1 folder and import the AAF to a bin. If you put new MXF in the MEDIAFILES Folder Avid sorts it out while starting up, as soon as you import the AAF the media is online. you don't need to move the AAF anywhere, only the MXF.
have to you tried to export a Quicktime DNxHD from redcine-x and (fast) import this file to Avid?
is there a avid media database in your Mediafiles Folder?
have you tried to delete your mediadatabase and restart Mediacomposer?
all the best,