On the upper part of the screen, you'll find all the social accounts you've connected to Postcron.
To select the accounts, you'll be working with you just have to click on them. The posts you create we'll be scheduled to the selected accounts, and only their posts will be displayed under the Future, Past or Calendar tabs.
It's very easy to identify them: Selected accounts will be shown in colors.
If you want to deselect an account, just click on it again. You'll notice that it will get greyed out, meaning that they are no longer selected.