Welcome to the forum.
The USB mic is not a MIDI device so it can't be selected as one.
Click the P button on your computer keyboard to bring up SONAR's Preferences and change the Driver Mode to MME.
Then you should be able to choose the Microsoft GS Wavetable synth as your MIDI device.
Here's some more info on using USB microphones with Cakewalk products:
http://www.youtube.com/watch?v=pcAtPmLtRns http://www.cakewalk.com/support/kb/reader.aspx?ID=2007012937 From the Cakewalk FAQ: 10) My USB Mic does not record.
USB mic's will work best in MME mode. Go to Options | Audio | Advanced and put a check where it says Always use MME Interface (or change the Driver Mode to MME). Close the program and re-open it. The Wave Profiler should run tests. You should now have more devices listed under MIDI Devices and Audio Drivers.
You may not be able to monitor that Mic while recording unless you use MME drivers, unfortunately MME creates a delay or latency. If you are able to use WDM and still get a delay when using (Input Echo or Input Monitoring), check out this link for more info on Decreasing Latency.
A third option would be using ASIO4ALL drivers. You can try out ASIO4ALL from
www.ASIO4ALL.com These 3rd party drivers are a good solution for generic or OEM sound cards that have unreliable WDM drivers and/or nonexistent ASIO support. Once installed open Cakewalk and go to Options | Audio | Advanced and change the Driver mode to ASIO. You may need to go to Options | MIDI Devices and shut down the Microsoft GS Wavetable Synth in order for this to work. Try using the input echo button on your audio track.
Fog's USB mic set-up post:
http://forum.cakewalk.com/tm.aspx?m=1618556