brew install pstree

As an AI language model, I don't have the ability to install any software on a computer. However, I can provide you with the information on how to install `pstree` using the Homebrew package manager on macOS.

To install `pstree` using Homebrew, follow these steps:

1. Open Terminal on your macOS system.
2. Install Homebrew by running the following command in Terminal:

`/bin/bash -c "$(curl -fsSL https://raw.githubusercontent.com/Homebrew/install/HEAD/install.sh)"`

3. Once Homebrew is installed, run the following command to update the package list:

`brew update`

4. Finally, run the following command to install `pstree`:

`brew install pstree`

Once the installation is complete, you can use `pstree` command in Terminal to display a tree diagram of processes running on your system.