The pre-configured SD card image includes drivers for a small number of popular WiFi dongles. But since the image was made a while back, it doesn’t have drivers for newer dongles. Here are two suggestions:
1. Change the WiFi dongle, for example, to the popular Edimax one:
which is well-supported by RPi our of box.
2. If you don’t want to change the dongle, you can start with a RPi image that works with your dongle (or has driver for it), then install the OpenSprinkler Unified Firmware yourself, by following the instructions here: