Hi Quinn,
You will find a convenient installer of all required PEST tools (PEST, PLPROC and the groundwater utilities) in the donwnload section on our website. If using this installer after the FEFLOW installation, the required file paths will be set automatically.
If you have downloaded and installed PEST from the official PEST homepage (www.pesthomepage.org), you can manually define the installation locations in FePEST under Tools > Options.