Lena, it's a model 15, not a potted motor, but electric, so that would make it a 15-90.
A model 15 has the tension on the chrome face (end) of the machine. The model 15 came in several sub-models. One is with a belt driven motor, the 15-90, on with a potted motor (direct drive - no belt), the 15-91 and as a treadle, which was the 15-88. Singer made the model 15 machines for many, many years, so there are earlier and later models of it as well.
Have you found the manual? If not, go to this page
http://www.singerco.com/support/instruction-manuals and enter your model number (15-90) and download a free pdf copy.
It doesn't have a drop in bobbin. You thread the bobbin into the case which is inserted under the needle and under the slide plate. The bobbin case has a little lever on the side of it that allows you to remove it. They are not expensive to replace if yours is lost. I'd get one from sew-classic.net if I needed one.
Everything to do with the 15-90 and a 15-91 are the same,except the motors, so you can follow most 15-91 instructions . Here's a youtube video on threading that shows the bobbin and threading the machine.
https://www.youtube.com/watch?v=EgzXcwz7w18
Let us know if you need any more help. Generally speaking, someone here will know the answer (or where to find it) within a couple of days to most vintage machine questions.
bkay