Yes. The Log In module works with all themes. If you have a brand new Tiki, you should see the Log In module in the top bar, by default.
When you add the Log In module to a Tiki column, be sure that you've assigned it to the Anonymous group (so that you can see it when you're not logged in). Also, make sure you haven't configured the module to appear only for specific categories, sections, etc. Please see the docs for details.
If you want to add the Log In module to a wiki page (instead of it appearing in a column), you'll need to use the Module plugin. This plugin allows you to add a module (such as the Log In module) to a Wiki page. Please see the docs for details.