For version 6.5.5 and above:
Under the menu “Language” there is a menuitem called “Define your language…“
In the tab “Folder & Default” is a group called “Folding in code” where you can enter an “Open”- and a “Close”-Keyword.
For versions older than 6.5.5:
Under the menu “View” there is a menuitem called “User-Defined Dialog…“
In the tab “Folder & Default” you can enter a “Folder Open Keyword” and a “Folder Close Keyword”