Please check Window->Preference->Flash Builder->Editors->Syntax Coloring
In the tree, you should see settings to MXML and be sure check the Enabled.
Normally, I think it takes effect by default and not sure how the highlights should look like.
Hi Felix, thanks for the reply. The fx:Script tags are colored in green, but the content inside <![CDATA[ ]]> isn't. Sorry for the confusion.