To connect your Pinterest boards to Postcron click on the + sign button at the top right side of the screen, and then choose Pinterest.
If you're already logged into your Pinterest Profile, you'll be shown a list of the boards it manages. If you aren't, you will be prompted to log in. Make sure you log into the profile that manages the board you're looking into connecting.
Then, you just have to click on the + button next to each board to connect them to Postcron.
Postcron will request that you accept publishing permissions in order to post to your social accounts on your behalf. Make sure you accept all of them to avoid future publishing problems.