In order to use MovieFocus, you first need to install the GuiLib library. Instructions are given on the GuiLib Setup page. Note that GuiLib only needs to be installed on the client.

Afterwards, you can continue with downloading the actual MovieFocus mod from the official download page. Make sure that you download the main JAR and not the dev one! Then, just drop the downloaded JAR into your mods/ folder, restart Minecraft, and you're ready to go!

Please note that MovieFocus only enables manual focusing if you have installed the GLSL shaders mod and activated one of the supported shaderpacks. Take a look at the list of supported shaderpacks in order to find out whether your pack is supported or not. Feel free to open an issue or even start a pull request if your favorite pack is missing.