There are installation instructions in the Pacsat Ground station manual. It should work on Windows, Linux and Mac.
The distribution includes an EXE for windows and a jar file. Double clicking the jar file should run the program on all platforms. You need to make sure Java is installed. Note that Apple, in its infinite wisdom, has decided that you need to install the full Java Development Kit before being able to run the jar file from the command line or from a script.
You will need to enter your callsign to get started. If you intend to receive only and don't have an Amateur Radio callsign then use a suitable ground station name. You will also need to setup the serial connection to your hardware TNC or a TCP connection to a software TNC. Then point your antennae at the spacecraft and see what happens. You should be able to receive the directory and send requests for directory holes and files. If you compose a message, then it should be automatically uploaded when it detects the uplink is Open.