To display your Holler Box when a link is clicked, you must have Holler Box Pro active.
Create your box, give it the content and settings you want. In the advanced settings meta box, scroll to "When should we show it?" and choose "Link click."
Save and activate the box. Make note of the box ID, it is the numbers in the url when you are editing the box. For example:
The ID is 2054.
Next, create a link that looks like this:
<a class="holler-show" data-id="2054" href="#">Show it</a>
Change the data-id attribute to the ID of the box you would like to show.