As to where - Illuminations is in the center of the lagoon which is surrounded by the World Showcase. You walk around the lagoon as you visit the countries. It usually goes off at 9pm. You can see Illuminations from almost anywhere around the lagoon, as well as down the main strip towards Spaceship Earth (the "giant golf ball")...see this link to Disney for info.
http://disneyworld.disney.go.com/wd...umiNationsReflectionsofEarthEntertainmentPage
Wishes is "over" Cinderella's Castle in MK. The fireworks actually launch from a few locations, including the rooftops of Snow White and Peter Pan rides in the park and from behind the park in the service area. If you watch from behind the Castle by the Carousel, the fireworks go off all around you, which can be fun (an a bit terrifying), but is not nearly as good a show as watching from in front of the Castle. We did it once and don't need to do it again.
Now, if your question is where is the BEST place to watch these shows,well, that is a can of worms! Everyone's got their favorite. My favorites are:
Illuminations - right up front, directly across from America. If you stand around, just before 9pm the CMs open up the chain and let you go out on the platform which is right in front of the show. I've watched it from the bridge in France, by Canada and UK, by Mexico and China, even from America, and the front platform is by far my favorite.
For Wishes, I like to watch it from the front, right side of the Castle as you are facing it, b/c it is easy to get to from Tomorrowland, it is on front, and Tink flys right over your head....