If the transcript is provided to you in PDF format and can be read in Preview:
1. In Preview, highlight the text (Tools > Annotate > Highlight Text).
2. Make a screen-shot of the applicable portion (Cmd-Shift-4 then drag the cross-hair across the section you want and then release the mouse button)
3. Place the screenshot on your Keynote Slide - the highlighting done in Preview is retained in the image and the text can be stretched to fit.
Unfortunately Keynote, Pages, Text Edit, etc. do not have a Highlight Tool.
Another way would be to Copy the text and Paste into a Text Box on your slide then place highlighting rectangles over the text and adjust the opacity so the text can be read through it, but it isn't very pretty and it takes more time.
A third way to highlight text is to use a Pull-Quote technique: Select and Copy the text of concern from a text box of surrounding text or image of the text, then Paste the copied text into an additional text box to be enlarged and placed on the slide over the original text. Done right you can cause the pull-quote to grow from the original text position.
Good luck.