Well, here’s the thing: Depending on your settings, doing so may open up that new event’s details in a completely separate window, which used to be handy, but under Yosemite, that window unfortunately doesn’t get selected.
This means that you can’t just create a new event and start typing in a selected field—you’ve gotta go over and click on the title (or whatever) to begin adding in the details. It’s a small frustration, but for me, it was happening many times a day, causing much wailing and gnashing of teeth. Grrr, I say. Grrr.
In any case, there is a quick fix of sorts, so to check it out, go to Calendar> Preferences and click on the “Advanced” tab.
Once you’re there, deselect the “Open events in separate windows” checkbox, and you’re ready to go. What’ll happen afterward is that when you double-click to create a new event in Week or Month view, the event’s editable details will appear in an attached window, and your cursor will pop automatically to the title field. Whew!
If you’re using Day view, it shouldn’t matter which way you have this set. Whether you’ve got “Open events in separate windows” on or off, you should be able to just start typing after double-clicking to add an event. However, if you’re more of a fan of Week and Month view like I am, this’ll save you a bit of time and frustration. Yes, yes, I am irritated by very small delays. So sue me.