I was able to do this before - now for some reason, I am unable to do it again.
I want to create a new track that records MIDI, but outputs to a softsynth on that same track. I also want the softsynth to appear in the EFX bin in the Track view.
I can create a synth track with MIDI, and place the softsynth in the EFX bin, but the notes won't play through it.
What am I doing wrong?
 

I don't know what you're doing, but I always simply go Insert>Softsynth or in the Synth Rack click add synth.
Then you need to select whether you want a Simple Instrument Track or separate MIDI/audio output tracks.
 

Synths don't belong in the EFX bin.
 
Use Insert>Softsynth, or open the add track menu in the track view, or Insert Synth from the Synth Rack view.
 
A simple instrument track combines the MIDI input with the soft synth audio output.  Or you can split the instrument into separate MIDI/Audio tracks.  Neither method involves the use of the EFX bin.

pdarg, you may not have the input monitor buttons engaged. You absolutely can insert synths into the FX bin. In Cakewalks "Pro Audio" days, that is how it was done. There may even be advantages to doing it this way (although I don't know what).

While it is possible to add synths to the FX Rack, it has not been the recommended practice in years. The better approach is add synths to the synth rack by using one of the insert synth options.
 
To simultaneously record audio and MIDI requires separate audio and MIDI tracks. Here is the documentation for soft synth audio recording.

You might need to read my simple instructions for how to use a VST instruments in Sonar, see my signature. Your certainly not doing it the way most of us do. It's really very simple once you get the basics down. 

Why do you want the synth to appear in the fx bin? Is there something special you're trying to accomplish with it?

I want quick access to change the synth patch.
 

pdarg
I want quick access to change the synth patch.
 

Open it as normal from the Synth Rack.
Dock it in the Multidock. It's a click away then.
 

While I haven't used this technique in a while it is possible this technique was broken in Sonar in X3 yet, I believe that it's still possible to use because you are still able to pick a soft synth into a FX box.  To get it to play, I would suggest set the input to none set the output to the soft synth you have in the FX box.  I have used this technique, and I have quite a few have projects that are still set up this way and, they play fine.  Although, I would follow the advice that the other posters have published and use the Insert technique.