If you want to add tremolo to audio, or to a synth that does not have a tremolo facility the VKFX Preamp/Stereo Tremolo - VST & ProChannel available for the Cakewalk shop for 7.99 UK pounds (probably around $12 in the US store) is a good tool, as is the whole VKFX bundle. You can either use it in Prochannel or the effects bin depending on you preference.
 
If you want something with more features Melda Productions MMultiband Tremolo. This is capable of simple tremolo effects and much more. Modulating different frequency bands at different rates and depths can make it sound like a modulated filter or even a phaser at some settings.

You can also draw a tremolo waveform (along with several others) as a controller. The draw tool has several options other than freehand draw and line.

You can also create a good portamento by automating oscillator pitch and fine tuning the resulting envelope.
You can get even more creative by automating the separate notes of a chord to glide from any 2 chords.
 
You might have to use separate instances of the same soft synth to do this
 
Remember to insert a controller setting the pitch back to normal before the next note(s) begin.

there's a free vst with tremolo and panner I use a lot called SIMPLE TREMOLO, I've grab it on the web for free years ago. Google it.
it is ridicously simple and easy going. Sometimes panner does a panoramic tremolo that worksjust  fine.

Thank you for all the answers, guys! I appreciate it. For the record, I was initially requesting help on creating Portamento when I accidentally mistook it for Tremolo.
 
I was doing some research on Portamento and I came across this URL in Cakewalk Documentation:
http://www.cakewalk.com/Documentation?product=SONAR%20X3&language=3&help=Views.22.html#1540732
 
This is great, but I'm slightly confused. The screenshot in the middle of the page -- what is it from? It doesn't look familiar.

It is the Step Sequencer, uplink to this page is http://www.cakewalk.com/D...amp;help=Views.11.html

LOL you could spend a week just trying out the different suggestions offered. Can't say we aren't an enthusiastic bunch!