There are a couple of things that are wrong here. The first is that the script that you are writing is being executed on the SAM poller server, not directly on the server running XenApp. It may be possible to execute the XenApp PowerShell commands directly from the SAM poller host, if they support remote connections. If not, PowerShell remoting might be needed.
The second issue is the output. The format for a single line output should be:
Message: *some message string here*
Statistic: *some statistic value related to above message*
In your script above, you have multiple lines of Message, this could be causing you an issue. If you are expecting multiple lines of output (up to 10), then the format is like this:
Message.somestring1: *some handy string*
Statistic.somestring1: *some statistic value*
Message.somestring2: *some other string*
Statistic.somestring2: *some other stats value*
For more details on all of this, you should read the Configuring and Integrating Powershell document.
