You need to download and run the ChemNet installer.
If your computer does not immediately connect to the ChemNet wireless network, open up the Wireless Network Connection box from your system tray and connect to ChemNet. (N.B. These screenshots were taken on Windows 7, but look very similar on newer versions of Windows)
When you connect, you should see a pop-up telling you that additional information is needed to connect to ChemNet:
Click on the pop-up, and you will be asked for your user credentials. Please note that your ChemNet username is your CRSID followed by @ch.cam.ac.uk (not @cam.ac.uk). You can collect the password (or ChemNet token) from https://apps.ch.cam.ac.uk/ssms/collectchemnet if you know your Raven password and you have been registered in the department database.
If the installer downloaded above does not work properly, you might be interested in the full step-by-step instructions instead.