Check your onsenui.js
I actually think it is possible that the project page on Monaca may not have been updated to the latest onsen ui 2.0. That appeared to be the case for me.
It may say in the first line:
/*! onsenui v2.0.0-beta - 2015-11-11 */
You probably need an updated one like the one Andi Pavllo used
/*! onsenui v2.0.0-beta.2 - 2015-12-01 */
However the latest Onsen UI 2.0 on Github is actually
/*! onsenui v2.0.0-beta.5 - 2016-01-08 */
And you cannot just swap out the js you would need to install the whole package.
*Edit: I noticed after I installed the new package everything was working properly but when I tried the theme roller, it reverted back to the old tab bar. So its likely that the theme roller may not be updated.