If the installer didn’t immediately go into “migrating” mode (where it sits at "Installing) for an extended period – and creates Migration.log – then it couldn’t read the contents of the Plex shared folder -OR- something was missing.
Let’s go through the checklist:
- The Plex shared folder must be readable by everyone
- It must contain Library/Application Support/Plex Media Server
On DSM 7, Plex no longer has any administrative privileges . Because if this, you must ensure the new DSM 7 username (PlexMediaServer) can read it. If it can’t read the shared folder then it won’t detect the data and start migrating it.
Since you have already installed the package , even though it didn’t detect the Plex shared folder, we can use that to our advantage 
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Uninstall the package with the ERASE option. (this removes the empty DSM 7 structure
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Control Panel - Shared Folder - EDIT the Plex share
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Follow this FAQ segment – “Scenario 2” to set the permissions for all files & folders in the “Plex” share to be Full Control by “PlexMediaServer”. This will take some time to complete (depends on how much metadata there is to process). You’ll know it’s complete when FileStation returns to normal . (Don’t worry if you’re installing on a different volume than the Plex shared folder… the FAQ needs updating)
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Now, with all the permissions set, reinstall the DSM 7 Plex package again.
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If you’ve done everything correctly, it’ll go right to “Installing” and sit there.
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After a few minutes, (Using a separate browser tab to connect to the Syno) you’ll be able to see “Migration.log” and follow the progress.
