The issue is that the process will execute asynchronously. The only way to get the value back from the process is, instead of using it as an output param, to send it as a message using a script task in the process that you will retrieve with client-side code on the page.
Basically, at the end of your process, you'll add a script task. In that script task you'll get the param value, then send it as a message (as outlined in the article). Then, in your page code, you'll listen for that message, check to make sure it's the message sent from your process, and then read the value and do whatever is needed with it.