Hi Dan,
Media Composer 5 only reads HD captions from AAF and MXF files with a SMPTE 436M track. It will not read any HD captions from a .mov, regardless of the codec used.*
You can use MacCaption to import the .mov HD closed captions and then convert them to a AAF file for use in Media Composer.
* Pedantically speaking, there used to be a way for DNxHD to have embedded VANC captions in a .mov container, but that method was extremely limited and it was deprecated by Avid when MC5 was released.
Best regards,
Jason Livingston
CPC Closed Captioning