Conversations will only appear for jobs where there is a quote between you and the other person. When a quote is won all conversations will close. The conversation will remain open until the job is completed.
Follow these steps
Log into your account: .
In the side navigation bar, click 'Messsages'.
In the conversations sidebar, find the conversation you want to view and click it.
You can enter a message into the message box to message the other conversation participants.