I found this out today and it blew my mind. First, go and try to highlight text in PowerPoint like how you can highlight text yellow. You will find there is no highlight text option and even you try to go to advanced Font options, nothing will be there. It must be a design constraint that would affect the way PowerPoint works, but it just seems mind blowing that one cannot highlight text.
Here is the official Microsoft PowerPoint Support page giving a work-around via a filling a box with color and dragging that over text: http://office.microsoft.com/en-us/powerpoint-help/highlight-text-HP001149298.aspx