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 buy 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 Zendesk calendar to your company's Google calendar. You can do this by clicking on the calendar icon on the left and then the "Connect to Google Cal" button at the top right.