Welcome to the installation guide!
To install the plugin you need to login to JIRA as a system administrator, and in the Administration menu \ Add-ons find our plugin by name: Calendar for JIRA and install it.
After installing plugin you will automaticaly get 30 days trial period. After that time you have to request a new license key. See instruction below.
To get your license through Atlassian Marketplace you must request trial key or buy commercial license in JIRA\Administration\Add-ons menu.