Download PSI from their website: http://psi-im.org
Install PSI and setup your first account.

Setup a new account. These instructions will work in both the Windows, Mac and Linux version of PSI.
Setup a new account
Enter the same username and password that you used in the mobile version.
Ensure that
Ignore SSL warnings is set.
Ensure that the priority is set to
12
You are now setup and can connect. Next we will ensure that you can join a group chat. By default you join
"opentoall"
One last thing to setup to make sure that when you leave your computer, messages are redirected to your phone. Make sure that the away priority is set to
9