Hi.
I'm still on OMV 2.0 and tried updating plex like mentioned in the "setup info" tab.
After this my plex-service was gone.
So I removed plex an did a new install (thx god my database was still ok), but was on version 1.9xxx.
I searched in the plex forum and found this script which compiles the newest plex version to a usable version for wheezy systems:
https://github.com/FozzTexx/plex2wheezy
copy the created .deb file like mentioned to /var/cache/apt/archives and execute dpkg -i plex(hit TAB and then ENTER)
after this you have the latest plex version on your old OMV-System