1. From the Quickstart Menu - select Keybaord and click 'Choose'. 

2. Create a new track by clicking the + button at the top-right of the Channel Strips area.

3. Click on the empty Instrument input to insert a new software instrument plug-in.

4. Select AU Instruments/Spectrasonics/Your Instrument/Stereo/

4. The instrument's interface will then appear.

5. If you need to show your instrument interface again, click the center section of the input on the channel strip.

Omnisphere Mainstage Template

Keyscape Mainstage Template

Trilian Mainstage Template

Stylus RMX Mainstage Template