When you set up a project to build tocjs output, you will likely want to pull source files from two separate locations. The tocjs plugin has a basefiles directory that includes HTML, JavaScript, image and CSS files necessary for the tocjs plugin to work. The tocjs transtype itself generates a JavaScript file (toctree.js) that is used in conjunction with the files in this basefiles directory. As you can see in the sample ant tasks in demo/tocjs/buildsample.xml.

You will likely want to maintain another basefiles directory along with your DITA source files and copy these into your output as well. If you look in the tocjs sample project there is a different basefiles directory. This contains the CSS file related to the DITA content and the frameset.html file. These are specific to the project, while the files in demo/tocjs/basefiles are for any project using the tocjs plugin.