The easiest way to find your version is to look in the server under Control Panel > Programs and Features. There should be an entry named "OpenScape Xpressions". The version should be indicated there. I'm on V7 R1.5.18409. If you have an older version, it could be named differently from the Siemens days.
Application Builder is where the trees are built.
Mailboxes are configured in the WebAssistant. If you don't have a shortcut, it should look something like this:
You can normally access this from any PC that can access the server. From there you go to Server Settings > User administration to see all of the mailboxes configured.
There's another application called "Monitor - System Logging" that defines the basic setup of the system. You can break a lot here, so take care! One important thing you can see here is how many lines you have configured. The system is licensed for the number of concurrent "calls" that Xpressions can handle at a time. This is under Preferences > ipApl > Advanced Settings > Device. This can also be seen in the License Manager.
Unify does offer a really good class for Xpressions and the Application Builder. It really helps if you want to get good at basic system config and building menu trees.
I just use Xpressions for voicemail and applications. Xpressions can do a lot more, to include faxing.
LoPath
Maintain HiPath 4000 V5 & V6, OpenScape Xpert V4 & V6, OpenScape Xpressions V7, OpenScape Contact Center V8, OpenScape Voice V9