Installing the Calendar app on your Zendesk is easy. First you'll need to go to the Zendesk apps marketplace which you'll find by first logging into Zendesk and then clicking the cog at the bottom left side of the screen, then click on "Marketplace" under "Apps" at the top.
Next, do a search for calendar and click on the Calendar app...
You'll then be able to buy the app by clicking the buy button and following the prompts. Note that you won't be charged for the first 14 days while you're in the trial period.
After that, you'll need to enable the app by clicking the new calendar icon in the Zendesk menu on the left of the screen, then enable the app by clicking "Enable App" and follow the prompts.
Great! You've finished installing the app and you're now right to start adding events to tickets.
The last optional step is to connect your calendars in the app to the Google calendar of your choice. You can do this by clicking on 'manage calendars' at the top left like this...
...and then on the right click the "Connect" button and follow the prompts.