To add custom snippets you need to edit the zen_settings.js file. You can edit it from Aptana directly or with other editor. Please note that the location is not the one from where you installed it. To know its location, right click the zen_settings.js file in Aptana, go to Properties and check the Location field.

Now open it with your editor of choice, and search for the HTML snippets section. Go to the end of it, add a comma to the last item and add yours. It may look like this:

I hope that helps.

Regards, Gonzalo González Mora