There are two ways to get the connection info into Mumble - first, entering it manually as shown in the top left of this page.
Alternately, you should be able to click the following URL - mumble://mumble.iswt.io:64738/?version=1.2.4 - this should launch Mumble automatically and attempt to connect to the server. If this is your first time connecting to the server, you will be prompted for a username.
Here are a couple of helpful tutorial videos that demonstrate how to connect your client up and start talking
Push-to-talk is the recommended way to use Mumble. This means that Mumble will not transmit your audio unless you're holding down the Push-To-Talk key. If the key isn't held down, nobody hears your voice or any other noise that would otherwise have been transmitted through your microphone. You may be familiar with voice-activated audio from using Ventrilo or TeamSpeak - where simply talking above a certain volume will transmit your voice. This is not recommended for Mumble, as Mumble has a very high sensitivity and requires a significant amount of tweaking and configuring to get to a level that most people are comfortable with, and which doesn't end up transmitting breathing or ambient noise in your surrounding area.
Once Mumble is running, go to the Configure menu option, and then select Settings.
After clicking on "Settings" a control panel will come up. Make sure that "Audio Input" is highlighted. Click on the drop-down arrow next to "Transmit" and select "Push To Talk".
To bind a key to be able to use your Push To Talk setting, head over to the "Shortcuts" tab, Click the "Add" button, and some options should show up in the middle column. Under the "Function" field, click the drop down arrow and select Push-to-Talk.
Double click the "Shortcut" field. You will see "Press Shortcut" appear - at this point, you should press the key you want to have for your Push-To-Talk setting. After pressing it, it should appear in the field preceded by K: - when you see this, press the "Apply" button, and then press the "OK" button.