Below is a more detailed picture of the slots where you insert the memory. In the circled section, you can see one bank the locking switches open, and when in the closed and locked position.
I have also circled the area that has a guide notch. You will notice that it is not in the centre of the memory slots, hence the memory can also only be installed in one direction. Take the memory sticks and firmly insert them straight into the slots, taking care not to twist or bend them. Once they are in position, you will notice the locking switches moving inward toward the memory. Just push them all the way to lock the memory into place.