Put the [[collapsible]] and [[/collapsible]] tags on their own lines or the parser will not recognize them.
By default, the show link says "+ show block" and the hide link says "- hide block". For longer blocks, add the hideLocation="both" option to show the hide link at the bottom as well as the top of the block when it's shown. Other values for the hideLocation option are "top" (the default) and "bottom".
Finally, you can use the folded="no" option to show blocks by default, allowing the user to hide them if wanted. We use this for tables of content, for example.