The ability to highlight text in your presentation is not available PowerPoint 2007. Instead, create a text box with color for any text you want to emphasize.

Select the Insert tab. In the Text group, click Text Box, and then drag out a text box on your slide.
Enter the text that you want highlighted in the text box.
Now select the Home tab. In the Drawing group, click Shape Fill, and then select the fill color that you want to highlight the text with.with.