If you are using Studio One Prime all you get is the ability to record 2 channels at the same time. If you are using Studio One Artist or Professional and you are not using a Presonus interface you may need to setup the inputs in the Matrix. Here's how.
1. create a new song
2. in the "Song" menu click "Song Setup"
3. Now click the "Audio I/O Setup" tab
4. Across the top are the inputs you have available. Now click "Add Mono" so that the column on the left connects to all ports across the top. Make sure a "Blue" box appears in each connection and click "Apply"
5. Now click "Make Default" so that all future new songs are set up the same.
Note: this will only be for new songs. Any currently existing songs will need to be setup manually.