If I understand this and your related post, you are trying to use a Raspberry Pi to control a relay board using OS firmware? If this is correct then it would be useful to know the version number of the firmware (you can find this from the About menu in the web app). The GPIO Station functionality requires the latest Firmware version 2.1.6 (2).
If you are not on the latest Firmware then you will need to upgrade using the instruction on the opensprinkler.com support website (link).
In the latest firmware, you should see a new advanced configuration option when you select the settings cog of a station in the browser. You should see a GPIO Station option that lets you associate a GPIO pin with a station.
Let us know if that resolves the problem.