MC5 & MIDI Sound Files

Since you have MC5 you should have a synth called The Cakewalk Sound Center. (CSC) This synth came with around 150 sounds in it to start with.
 
In addition, in the cakewalk store, you can buy add on sound packs for $20 each that have more sounds and more instruments available in them. I added a piano pack and the world instruments pack.
 
Here's the direct link: http://www.store.cakewalk.com/b2cus/Category.aspx?CategoryID=83  this gets you to the CSC packs page.
 
In addition, you can also buy just about any other VST synth and add it to the program. I have Kontakt and a number of other third party synths in the system.
 
You also should have TTS and a few others (IIRC) in the program.

Under the INSERT command (top tool bar) use the INSERT SYNTH function and it should be in the list.
 
Choose it and it will load. On the interface that shows up, there is a horizontal menu tree.... simply select the main instrument on the left....such as BASS, then narrow that down in the center column, and the right side column will be a list of all the available individual instrument samples in that category..... double click and that one will load. The GUI remains, and if the sound is not to your liking, simply click another from the right side column or start over to change to something totally different.
 
As you purchase and install new sound packs for CSC, the new sounds will automatically populate the correct categories and become available.