Installation is automatic when you install the app. You can check on the status of the install by clicking on the "Settings" button in the top right-hand corner of the app and selecting “Installation” from the options:
Bundle Builder installation screen
The installation process will install a single template in your current theme. The file is called 'product.bundlebuilder.liquid' and will be in your 'Templates' folder. This file ensures customers are automatically navigated to your bundle page.
If you are using a new theme, or have deleted the snippet, you can reinstall by clicking 'Reinstall' here.