To message a seller simply tap on the Item you are interested and on the Item's screen tap the 'message' button. This will open a chat screen for you to ask about the details.
To contact the Campaign Champion, on the Campaign screen scroll down to the "Champion" section. Tap on the "Get in touch!" button to message them.
To message the Organisation Manager, you need to access the organisation profile. To do this, tap on the Organisation name or logo on a Campaign screen and scroll down to the "Manager" section. Tap on the "Get in touch!"