Here are some instructions on creating this effect:
First, create one text box and size for length of sentence.
Next, type in the sentence, and when you get to the word you want to appear
to "Fly In", use Space Bar to insert a large space, then continue to finish
typing the sentence.
Now, click on the Insert Tab, then click on Textbox, then position the
cursor in front of the empty space is where you want the word to "Fly In",
then, without letting go of the cursor, drag to create a new textbox on top
of the first.
If needed, reposition and resize for desired position, then type in the
word(s) you want to "Fly In".
Next, on the Animations tab, in the Animations section, click on Custom
Animations, and the Custom Animations dialog box will open to the right of
the window.
With the second Texbook selected, click the Add Effect button, point to
Entry, then click on Fly In.
To preview the result, simply click the Play button at the bottom of the
Custom Animations dialog box, and the word will appear to fly in to the
"empty" space in the sentence.
This process simply layers the second Textbox on top of the first. You
prepare the first with the empty space for the word(s), then layer the second
Textbox on top of that empty space then apply the desired effect.
It may take a bit of messing around with in order to get it just right. Good
luck.