I ran across this issue today in an AE project that I converted to the latest update of AE 15.0 on a Mac.
Trying to R click the unlinked file and selecting Replace Footage / File resulted in a greyed out AVCHD file. Next, I went into Finder and Navigated to the file by R clicking on the AVCHD file / Show Package Contents. Repeat for BDMV file, as metioned in the earlier post. Option dragging did not work for me, but I R clicked on the missing file one more time / Replace Footage / File and the file was available.
I guess AE needs a little help by manually opening the Package Contents in Finder. Kinda weird.