Here are instructions from the PreSonus Knowledge Base article on setting up a new MIDI device. Here is the link to our knowledge base.
Set Up Control Surfaces
In Studio One, a Control Surface is a hardware device that includes transport controls, faders, and other specialized controls. The control surface might use MIDI directly or via a special control layer such as Mackie Control.
To set up a Control Surface, do the following:
1.In the Options/External Devices menu (Mac OS X: Preferences/External Devices, click on the [Add...] button.
2.Choose your device from the predefined device list. Set this to New Control Surface if you do not see your device in the list. If set to New Control Surface, you may wish to type in a Manufacturer Name and a Device Name in the appropriate fields. This makes identifying the Control Surface easier.
3.Specify the device to which the Control Surface is sending and the device from which it is receiving via Studio One. Select your MIDI device driver name from the drop-down menu for both Receive From and Send To.
4.You do not need to specify the MIDI channels your Control Surface should use, as control surfaces use alternative protocols, such as Mackie Control, to communicate with Studio One.
5.Your Control Surface is now ready for use in Studio One.
For more information on using Mackie Control devices with Studio One, see Mackie Control.