Inverness - you're just along the road from me.
I'm not sure whether you meant what the other poster suggested - i.e. highlighting in preparation for a cut-and-paste operation - or whether you meant highlighting as in 'drawing over it with a highlighting pen'. There's no easy way to do this directly. Of course you can change the colour, font, text size, style etc but if you really want to highlight then the only way to do this is UGLY.
OK - make sure you're sitting down before you read Tim's ugly way of highlighting text in Pages...it's bad...
OK - highlighting in Microsoft Word is not a problem and, guess what, Pages can display a document correctly if you import it and it has highlighted text. Now you can see what's about to come, can't you. I said it was ugly. I keep a 'prototype' document in the My Documents area of Pages that has all sorts of stuff in - superscript and subscript, footnotes, symbol font and some highlighted text. When I want one of these effects in a Pages document I copy and paste it from the prototype document into the document I'm working on. I can backspace through the highlighted text until all the unwanted words are deleted and then start typing and the text all comes out highlighted. In fact, you need to paste the prototype highlighted text into the middle of some non-highlighted text in the document that you're working on otherwise you can't get out of highlighted mode. Same for superscripts, subscripts etc. Ugly workaround but it *does* work.
Tim