Type the text you're gonna dissolve on it's own separate layer. It's good to name this layer so you don't get it confused with other stuff in your movie.
Step 2:
Convert your text into a movie clip symbol. Either click Insert> Convert to Symbol, or press F8. Name it something like "dissolve". Double click on it's name in the Library (Ctrl-L) to edit it.
Step 3:
Okay, in your symbol's timeline, create a number of layers equal to the number of letters in the to-be-dissolved word. Name these according to the letters. Check out Figure 1.1.