I'm running Sonar X3 Studio on a Windows 7, 64 bit PC.  Plenty fast and with lots of memory...
 
A simple experiment - Insert a software synth track.
I open a new project.
I insert an audio track and I put a soft synth (Cakewalk Z3TA+ 2) into the FX bin.
I insert a MIDI track and set the Input to my MIDI controller (Oxygen 25 or AKAI MPK61; both are selected in the MIDI Devices section of the Preferences menu) and set the Output to the soft synth.
I get BUPKIS
When I play the little keyboard at the bottom to the soft synth window - Loads of BEAUTIFUL SOUNDS - clearly there is an issue with MIDI being sent to the soft synth within Sonar.
I'm not seeing any input activity on the MIDI track meters when I activate the keys on the Oxygen 25...However, if I set the MIDI track output to anything else than the soft synth, for instance if I set the MIDI output back to the Oxygen 25, I do see activity on the MIDI track meters.  I totally don't get this one...?????
 
Try again
Reboot the computer - repeat exactly the above
I get BUPKIS
 
Try again
I open a new project.
I select a soft synth (Cakewalk Z3TA+ 2) as a simple instrument track.
I set the Inputs and outputs as appropriate
I get BUPKIS
When I play the little keyboard at the bottom to the soft synth window - Loads of BEAUTIFUL SOUNDS - clearly there is an issue with MIDI being sent to the soft synth within Sonar.
m not seeing any input activity on the MIDI track meters when I activate the keys on the Oxygen 25...However, if I set the MIDI track output to anything else than the soft synth, for instance if I set the MIDI output back to the Oxygen 25, I do see activity on the MIDI track meters.  I totally don't get this one...?????
 
How about this...
Load the same configuration into Bidule
Oxygen 25 MIDI input -> Z3TA+ 2 -> M-Audio Fast Track Pro
Loads of BEAUTIFUL SOUNDS - clearly there is not an issue with MIDI being sent to the soft synth.
 
Please help a brother out here...

Turn on the Input Echo button

Input echo is on...

Maybe try adding the synth and tracks using the synth rack.

Open the synth rack (alt-9 I believe)
Click the plus button and select your synth
In the dialog that appears make sure it is creating the midi track and whichever output selections are appropriate.

Then after verifying a sound is loaded, click the input echo on the MIDI track and of course, make sure the midi track's input is your keyboard.

Other items are taken care of.

As a thought, did you set your midi track's output to it virtual synth's input?

I have tried using the Synth Rack to insert the "simple instrument track"...nope

Do other synths work?

Please try doing the tutorial in the reference guide.

Access direct from the PDF guide or the Help menu. Find the guide in c:/Program Files/Cakewalk/Sonar Platinum (or whatever your version is) Sonar Reference Guide.pdf

Not blowing you off but it will give you step by step instructions.

You are not inserting the synth correctly.  The synth should be associated with the MIDI track, not the audio track.  The audio track's input should be the synth output.
 
Use the browser to drag your synth to track view and when the insert synth menu come up make sure midi source and first synth audio output and midi output are checked.

A couple of suggestions.
 
Check which channel the MIDI controller is transmitting on. Then dig into the preferences of the soft synth and make sure the synth is looking for input on that MIDI channel. If your controller is spitting out notes on MIDI channel 5 and your soft synth is listening to MIDI channel one, you're gonna have a bad time.
 
I know you said you have input echo on, but just to verify, arm the track and record some notes. If you get sound when the track plays back, you've got some kind of MIDI echo issue.

This matters only when inserting an instrument in the synth rack, but in the instrument GUI's header, check that "Enable MIDI In" is enabled in the VST2 drop-down menu.