To connect your Twitter accounts to Postcron click on the + sign button at the top right side of the screen, and then choose Twitter.
If you are already logged into your Twitter account, you'll be shown your account on this pop-up window. However, if you aren't, you will be prompted to log in. Make sure you log into the account you want to add.
Postcron will request that you accept publishing permissions in order to post to those social accounts on your behalf. Make sure you accept all of them to avoid future publishing problems.