Mouseover a particular date in the calendar. This open a contextual pop-up with a set of options for you to select and add.
Mouseover a particular date in the calendar and you can find a set of options from this choose Task radio button. This open a New Task contextual pop-up.
To add a task :

You can see your tasks getting listed in the calendar view along with the task count.
When you mouseover a particular date in the calendar. You can find a set of options from this choose Milestones radio button. This opens a New Milestone contextual pop-up.
To add a milestone :

( Note : Default selected flag is Internal )
The milestone added spans as a long thin green bar between selected dates.
When you mouseover a particular date in the calendar. You can find a set of options from this choose Event radio button. This opens the New Event contextual pop-up.
To add an event :

The scheduled event will get listed along with the repeat interval and meeting count.
Mouseover a particular date in the calendar and you can find a set of option from this choose Bugs radio button. This opens a File a Bug contextual pop-up.
To add a bug :
You can see the newly filed bug in Bugs tab under All Bugs view.
