Method 1 : While being in the environment, clicking on an object with the "MIDI out" tool will assign that object to the currently selected track.

Method 2 : If you open an Arrange window and an Environment window, go to the Midi Instruments layer (or layer where you want to assign a Multi-Instrument to a track). Make sure that on the Multi-Instrument, no individual channels are selected. With the tracks from the Arrange in view and the Environment window in view, drag the Multi-Instrument to a track. That track and the next 15 subsequent tracks are assigned to the Multi-Instrument you dragged (starting at MIDI channel 1 through 16). If there aren't enough tracks, Logic will ask if you want to create new ones. If on the Multi-Instrument you select a single MIDI channel, then dragging the Multi onto a track will assign just the one track that you drag it to.