The VpnCilla client is a paid for application from the Google Play store. The Play Store is available on some Chromebooks - more information can be found here. The trial version should be installed to check it works on your Chromebook before paying for the full version.
If you are using a University enrolled Chromebook, you will not currently be able to download VpnCilla on your University log in.
Once you have set the client up for the first time, you can connect easily in future: