To join a channel on patter.chat (pnut.io) configured for public use:
/join #pnut_ID:monkeystew.net where ID is the id number of the channel on pnut.io
You can get the id number by looking at the URL when viewing the channel in patter.chat
By default any messages you send will be posted as the matrixbot user on pnut.io with your display name. You can however authorize pnut-matrix to post as your account instead.
To link your Matrix ID with your pnut.io account:
A listing of active public chat rooms on pnut.io can now be found using https://patter.chat.
You can also see a list of channels already bridged using the room directory using Riot.im. Type monkeytew.net in the Matrix dropdown search for #pnut_.