How to insert video into a still

As Carlos said.

So, put the picture on track 1, then then video on track 2, and with the video track selected, use the 'Scale' property to make the video the right size. It's going to be in the 25-30% range, then move the scaled video to where you want it, in the lower corner.

You can just scale and reposition your video 

from the properties panel if that fits correctly, or:

Draw a shape around that area

Apply Track Matte Key to the video

Choose the shape video track number in 'Matte:'

scale and reposition your video using the properties panel