Interleave Stereo/Mono Button Missing From Softsynth Instrument Track Inspector?

Split the instrument track and use the interleave on the audio track associate with the synth. If you just need better control over the stereo output of an instrument track use Channel Tools.

alewgro
Thanks guys, great answers here, and yep it works splitting it out.  Am I using it for the right reasons?  I am trying to make a stereo synth bass patch to be mono in the mix.   I remember back in the day you can actually MAKE mono tracks, but I can't find that option anymore.
 
Thank You,
Alewgro

Select the clip and choose Clips > Convert to Mono. However I believe this is a simple addition, so if both channels are hitting max headroom, you'll get clipping. Use Process > Apply Effect > Gain and lower the gain prior to converting to mono.
 
You can create a mono track when recording by choosing only the left or right channel of an interface stereo pair.

When converting a stereo synth bass track to mono, I usually just flip the interleave button to mono and then change the track's synth input accordingly. For each audio output the synth has, you can select mono left, mono right or stereo. I change it to mono left or right. 
 
However sometimes the synth bass will have a lot of good stereo information in the higher frequencies which is a shame to lose by converting it to mono. But you still want to keep a nice centered bass. So what you can do is insert a plugin which centers frequencies below a certain threshold and keeps everything above it in stereo. Tone Projects make a free VST called Basslane which does this perfectly. Just slap it onto a synth bass track and set the frequency threshold to something like 200-300Hz(or lower if you like). Now everything under that threshold will be centered in mono, leaving the higher frequencies with their stereo width. Stereo synth bass sounds are very popular these days but if the lower frequencies are wide then the track loses a lot of its oomph.