This will probably be the first window you will see after starting the client. The window appears after successful connection to a metaserver that holds the list of available netrek servers.
Here is how it looks:
There are several types of server settings, such as Bronco, Hockey, and Chaos. Most of the servers you'll see these days are either Bronco or Hockey. Bronco is the most popular type of Netrek, so go on and click on some Bronco server to connect to the game.
In order to join the game to play you should point on one of the servers and click
the left mouse button.
If you wish to observe the game instead of playing - click the right mouse button.
Clicking on one of the yellow links will open your browser to the specified website.