Maggie, don't get frustrated! Keep in mind too that the lower abdomen may be the last place you lose fat. But, if it is lax muscles, doing core work will help.
The easiest way to get you to activate your transversus is this. Stand relaxed and let your belly totally hang out. Now, pull your belly button in, like you are trying to zip up tight pants. If it is too hard in standing, get on all fours and let your belly hang, then pull your belly button up toward your spine.
A posterior pelvic tilt is where you lie supine (on your back) with your knees bent. Rotate your pelvis so that you are pulling your pubic bone up toward your ribcage and flattening your lower back into the floor.
Once you are in this position, the bridge part occurs, where you extend your hips and press the front of your pubic bone toward the ceiling, maintaining that pelvic tilt (ie do not let your lower back arch!).
For many women, standing in an anterior pelvic tilt is very common. These people need to work on consciously maintaining a more posterior tilt, bringing them to neutral. (Although there is frequently more to it than this ... with glutes not working, TFL being overactive and tight, hip flexors, etc) This in turn will lessen the appearance of a "tummy".
If you search around here and on T-Nation there are some good things posted about anterior pelvic tilt.
Hope that helps!