Logic Pro recognizes all MIDI devices set up in the Audio MIDI Setup (AMS) utility, the integrated audio and MIDI configuration tool of Mac OS X. You can find the AMS utility in the Applications/Utilities folder. For more information on use, see the AMS Help.
The Library tab displays all MIDI devices found by the AMS utility (separated into MIDI channels, if multichannel devices) when an External MIDI track is selected. You can simply select the MIDI device/subchannel to assign it to the track.
You can configure external MIDI devices in the Inspector. For more information, see Standard Instrument Objects, Multi-Instrument Objects, and Mapped Instrument Objects.