Hmm - I can't answer re that specific reg as a search on google for "mc78ld5acp" brings up this page! So now you have managed to get your post to the top of the google search engine, I suppose we have to answer this *grin*
I'm assuming it is a 5V regulator? If yes, well you can't use it as a 3V regulator. Regulators come either as fixed (7805, 7812 etc) or variable (LM317 et al).
If you want 3V, you could use a fixed 3V regulator or a variable regulator and set it with two resistors to give 3V.
Regulators are very easy. You put in any voltage (within reason, don't go above 25V) and out comes a nice smooth 5V. Or 12V. So the input could be a car battery that changes between 11V and 14V but you always get 5V out of the regulator. You can't do that with a voltage divider.
The capacitors don't affect the voltage. They are there to smooth the voltages. Generally, just follow the recipes. I tend to put 470uF on the input, 22uF on the output (both near the regulator), and then right next to every chip on the board, I put a 0.1uF.
There are different ways of describing values. uF (microfarad), nf (nanofarad) and pf (picofarad). 100nF is the same as 0.1uF.
If VDD and VCC are confusing, just use Ground and 5V and everyone will know what you are talking about.
http://encyclobeamia.solarbotics.net/articles/vxx.html
There are different types of capacitors. Electrolytics tend to be bigger value ones. Ceramic, mica, greencap are smaller. Tantalums are more expensive but they have some advantages compared with electrolytics.
Re "I really don't use capacitors often enough." - capactitors are fundamental parts in electronics. Think of them as containers of water. Small value ones might be cups, middle sized ones buckets, and big electrolytics are rainwater tanks. (Supercaps are reservoirs!). Say you are an IC and you have just switched on a LED. That is a bit like grabbing a drink out of a cup. You want a drink and you want it right now. If you go to a tap and turn it on, the water takes a little while to start flowing. An example is when you drive over a fire hydrant and the geyser takes a while to get going. So the 0.1uF "decoupling capacitors" or "bypass capacitors" are like little cups of water sitting right next to the IC ready to supply any extra energy quickly when it is needed. Of course, you will need to refill the cup, and this is done more slowly from the big bucket down at the regulator, the 22uF capacitor.
As for your original part number, can you just double check that or give a pointer to a datasheet?
