Hi L.,
I am a massage therapist and took a moment to watch the utube link that someone sent to you. Those exercises look helpful and actually seeing someone else do them helps a lot. When you are trying to find the medial pterygoid muscle, think of the area that peanut butter tends to get stuck and you'll find the place more easily.
I am familiar with a similar exercise routine that is a little simpler, although hard to describe. You open your mouth slightly, just to a relaxed position. You then move your jaw in each direction (up, down, right, left) while resisting the motion with your hand as shown on the video. It may be easier to do a up/down sequence for a while, then switch to a right/left sequence. I've seen someone do right/down/left/up but it takes more coordination than I have!
There are several other muscles that can be factors causing temporolmandibular joint disfunction, including the lateral pterygoid (located in front of the ear), masseter (chewing muscle) , sternocleidomastoid (resembles a cord that inserts just below the ear and reaches down to attach to two areas, one on the sternum and one on the clavical) and trapezius (shoulders and neck).
Have a blessed day,
B.