How do I open up my DVD drive on my Lenovo Idea Pad White 320?

There might be a button on the keyboard to eject it

Or you can open "My Computer" from the start menu in Windows and right click on the optical drive there to eject it.

In the start menu, look for "this PC", double click, find the DVD drive, right click and select "eject"

I see that at least some are sold with a dummy slot. If you provided your full model part number I could have confirmed it. If no button or little hole, it is likely a dummy slot.

In Windows Explore, This PC level has a list that includes the C: Drive storage. If there's also a D: DVD RW, there's a drive and double click or right click gives an option to open it.
If no drive, it isn't there. There may be a connector inside for one and an available space designed in, and you generally would also want a slot cover. The simple way is buying an external USB optical drive and only use it when needed.
Desire for a lighter weight pc and faster internet connection is obsoleting dvds.

At Curry's in the UK:

Disc drive No - dummy drive

Amazon UK model 80XR0081UK shows NO DVD DRIVE

This grey one at Amazon US has one, and see the button:

Model 80XL003HUS has one

I believe USA and Canada models have one, and other countries may not.
http://psref.lenovo.com/Product/Lenovo_Laptops/ideapad_320_15_320_15_Touch