The only non-powershell statement is the echo. I would recommend logging a transcript file that you can then review for errors. Certutil will write to stdout so that will be logged into the transcript.
But msiexec will not write to stdout, so you should add logging to the command line parameters.
start-transcript -path $env:systemroot\temp\MyScript.log
certutil -addstore "TrustedPublisher" Mediaocean.cer
msiexec /i "https://moinstaller.mediaocean.com/MOInstaller/DDSShell64/DDSShell64.msi" /qn /norestart /lv $env:systemroot\temp\MyScript-msi-1.log
msiexec /i "https://moinstaller.mediaocean.com/MOInstaller/MOFramework/MOFramework.msi" /qn /norestart /lv $env:systemroot\temp\MyScript-msi-2.log
stop-transcript
Then if something doesn't work, you have the 3 log files in C:\windows\temp that you can review for errors.