This is a project file with a geometry nodes modifier setup, not an addon.

To import it into your project:

 - Open the asset project;

 - Select one of the flame objects and copy (CTRL+C);

 - Then open your project and paste (CTRL+V). It automatically imports required node groups.

To add flame to an object:

 - Select the object;

 - In modifier settings add Geometry Nodes modifier;

 - Select Flame Generator node group:

Cycles

In cycles you may have black artefacts like this:

To fix it increase Transparent light bounce count under Light Paths tab:

Custom Color

To set a custom color gradient for the flame open the "Realtime Flame" material node tree and connect the Color Ramp to the Color socket: