How can I set Dimension pro 1.5 for composing orchestra?

DimPro is only 4 voice timbre  - one for each element.  It is not a GM synth that will fire from all 16 midi channels at once  There is a page where you can turn on multi-timbre, but many patches use more than one element. For an orchestra, you'll have to use multiple instances of DimPro.  If you go through the patches, you'll find most of the instruments that make up an orchestra.  There are also libraries to expand your orchestra palette.  Check the Products page - I think there is a sale right now.
 
It is the same for doing drums w/ DimPro.  One element could be the kick, one the snare and the other two hat open and closed.  The next instance of Dimpro could have the cymbal and 3 toms.  Most samplers handle that like a drum machine - one midi channel with different notes playing the separate drum sounds.
 
You can do it, but it will take several dimpros open at once.  The good news there is that dimpro is pretty light on the CPU.
 
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You might also want to look at downloading the free Kontakt Player, from Native Instruments, or Reaktor Player .
 
They both come with a very small subset of sounds from their full-version bigger brothers, but for free they are not bad at all, and you might find some decent sounds there.  
 
The full version of Kontakt is highly regarded as one of the best sample players on the market, and the free Kontakt Player's included sounds are nice.  The same holds true for the Reaktor Player, though it is not really orchestral in nature - it still might have some sounds you could use.
 
Here is a link to their free players: 
 
http://www.native-instruments.com/en/products/free/
 
And, there are other synth components included with Sonar, that might help you with your class projects - which ones you get varies with the different flavors of Sonar - Producer having the most.  TTS-1, Rapture, SI-Strings, etc.
 
I hope the above helps you with your goals, 
 
Bob Bone