Add Issues Via Calendar
To add issues right from the opened Calendar's page select a day or a period and in the opened dialog select an option "New issue" or "Existing issue".
If start and end date fields (that were set in Calendar's admin menu) are in the create issue screen scheme you can check that the selected dates in the calendar are populated in the fields. If you are using Date picker field type (as in example) issues are placed for whole day and the finish date is set 00:00 or 12:00 AM of next day.
Notice that created issue doesn't appear in the calendar immediately. Calendar takes some time to refresh the page.
Add Existing issue
"Existing issue" opens search bar where you can start to type issue key or issue summary to limit the search results. The search is made within issues that respond to calendar's main filter (Calendar's Cog / Main / Display). Select an issue from the dropdown list and click "Put issue on calendar".
The issue will appear in the calendar after calendar's automatically refreshes.
Fill in Start and End Dates in Issue
Issues can be displayed in Calendar automatically if Start and End date fields are present in issue view/edit screen and are filled in.