At the moment Messagenet Talk app for Android is available as a Private Beta, which means that you can use this app only prior registration.
First of all you will need a Messagenet account (if you still haven't one). You can create one for free at the page: http://www.messagenet.com/registrazione/
Then simply sign up for the Private Beta using the same email address and mobile number of your Messagenet account here: http://talk.messagenet.com/androidbeta
You will receive by email in a few days the instructions to download and install the app.