Typically nothing comes up when Mir is applied if:
- Mir is getting used on a system that doesn't meet the minimum requirements-- namely a GPU that doesn't have at least 1GB of discrete VRAM for HD projects, or 2GB for 4k/high resolution compositions. Note that discrete VRAM means it is dedicated (like an AMD or NVidia card) instead of integrated (like an Intel card).
- It's getting applied to a large (>2k) composition; the Far Vanish/Near Vanish settings are built around assumptions that it's getting used in an HD project. Adjust the Far Vanish/Distance settings of Mir to get Mir to appear in your composition.