You have that a bit backwards because it’s not possible to grant permissions for “PlexMediaServer” until after DSM 7 is installed.
While the full process is here, which is what’s listed at the top of the thread:
The “Migration Instructions” in the post I linked has screenshots to help you along the way. This is a P.I.T.A process and thankfully won’t need be redone after this.
Let me give you the TL;DR
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Uninstall the DSM 6 Plex app
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Upgrade to DSM 7
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Reboot DSM again after upgrade
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Download Plex (dsm7) from https://plex.tv/downloads
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Manual Install – give it the SPK you just downloaded
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The installer in DSM 7 has a lot more interaction with you than it did in DSM 6.
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Expect it to complain / fail the first time. This is because username “PlexMediaServer” doesn’t yet exist.
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The installer will give you instructions, tailored to your system, in its pop-up messages as it gets to each point of validation.
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If you follow the Migration Steps, not missing the fine details – which there are a lot of – you’ll get through in the first attempt. When everything has been satisfied, it will sit there at “Installing” while it does everything needed to migrate from DSM 6 to DSM 7.
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The amount of time required will depend on how much media you have indexed in DSM 6. (one user had a 480 GB “Plex” share which needed 2 weeks to finish).
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Whatever you do, Don’t restart the NAS. Let it finish.
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You can always open a separate tab to DSM and look at the file “Plex/Migration.log” to see what it’s working on. When it’s done, it will tell you it “Completed: Clean /volume1/Plex” in the log file.
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After installation is complete, now you can go to each of your shared folders and grant permission for System Internal User “Plex Media Server” to read or read/write as you deem appropriate.
I’ll be here for a while if you have any questions.
