The plugin is built into the theme so after theme activation you will be asked to install it. You can always find it and install from the WP Admin Panel › Appearance › Install plugins section.
How to update?
After theme update, if new version of the plugin is available, you will be asked to update it (notice on top of the Dashboard).
If for any reason you can't see the notice, you can update the plugin manually.
To do that, please go to the WP Admin Panel › Appearance › Install plugins, find the Photomix plugin on the list and click "Update" right below it.
If you can't find the Install plugins link in the WP Admin Panel › Appearance section, all your plugins are up to date and you don't need to update anything.
After activating the plugin its settings will be accessible under the WP Admin Panel › Settings › Photmix page.