Field won't activate from theme sub-folder

When I add this plugin to my theme (as instructed), the Range option does not show up in my ACF options. I confirmed that I included it correctly by echoing a test phrase to my screen from acf-range.php, but it wasn’t until I removed it from my theme and then added it as a regular plugin that I was able to see the field.

https://ww.wp.xz.cn/plugins/acf-range-field/