If you selected the pack but don't see the retro Roblox-style HUD, work through these checks in order.
Make sure it's loaded as a shader
1. Confirm you have a shader loader. The pack runs through custom visual effects, so it needs Iris or OptiFine. Without one, there is no Shader Packs button in Video Settings.
2. Check the folder. The file must go in shaderpacks, not resourcepacks. If you put it in the resource pack list, it won't do anything.
3. Don't unzip it into nested folders. Leave a zip as it is. If you extracted it, make sure the pack's contents aren't buried inside an extra folder layer.
4. Select it and apply. Open Shader Packs, click the pack, then press Apply or Done.
If it's selected but still missing
- Reload the shaders. Switch shaders off, apply, then select the pack again and apply. This forces a fresh compile.
- Rejoin the world. Leave to the title screen and load back in.
- Disable other shaders. Only one shader pack runs at a time. Make sure this one is the active choice.
- Check for conflicting visual mods. Temporarily remove other mods that change rendering, then test again.
When the pack is working, the world should still look close to vanilla, with the 2008 client HUD on top. If the world looks normal but has no HUD, the shader isn't active yet.