This is a few days old, so I don't know if you figured this out yet, but I will post this anyway.
1. Make sure your add your username and password to this line:
<Account Username="" Password="" UseCustomServer="false" CustomServerAddress="" />
2. Add your hostname to this line:
<ComputerInformation Name="" Group="Default" />
3. in the <MonitoredServices> section add the line:
<Service Name="plexmediaserver" DisplayName="Plex Media Server" IsDaemon="true" DaemonType="UPSTART" Path="" StartParameters="" CanBeStopped="true" Enabled="true" />
If the service shows it's not started in Pulseway, then change DaemonType to SYSVINIT. a couple years ago there was a problem when Ubuntu switched to upstart. I haven't used Plex in a few years, so I don't know if they made a upstart version for Ubuntu. Newer versions of Ubuntu will run most sysv inits just fine and it doesn't hurt to try to see which one works.