What are you trying to do that isn't working?
To edit any existing highlights you would have to be in the Highlights tab in the synth editor, as I'm sure you know. When there:
+ Clicking on any existing highlight should display the current title and caption, allowing you to edit either.
+ To reorder the highlights, click on the one you wish to move and use the 'Move Up' or 'Move Down' links at the bottom of the editor.
+ To add new highlights, browse to the photo you want and click the 'Add Highlight' button.
+ Inversely, when you've got an existing highlight... eh... highlighted, you can click 'Remove Highlight' to get rid of it.
+ As always, remember to click 'Save' when you're done. Some people even prefer to save every time they change something.
If something listed here isn't working for you, perhaps you're having problems with Silverlight. What web browser are you using?