How do I place a chord on the right key with a note?

Well, there are software applications, basically specialized spectrum analysis apps that will sort out the simultaneous notes and give you the chord name and voicing. Some will even print a score and convert to MIDI. Most chord samples put the root at the MIDI note that calls the sample, so if you know the type of chord and know where the root should be you should be able to find a match without either software or a trained ear.
 
But if your other notes are sounding a chord that is dissonant with the sample, you are not looking at a mismatch in root pitch, but a mismatch in harmony, and just changing the sample up or down the scale is never going to sound right.

sock monkey
To help explain , first, are you a musician or a non musician creating music? 
This will speed up the process.. no point explaining things in musical terms etc.. 

I've been using Sonar x3 for over a year, but I'm still technically an amateur.
 
SLAR -- Thanks for the detailed explanation. It seems as though, although I had a hard time precisely explaining the issue, you hit on what I was trying to say.
 
I did consider layering the chord over three or four keys at once to blend itself in, but that would just make things more complicated. That's essentially eight notes playing at once.

OK, so I take it you have no musical training. I asked because sometimes we assume people don't know things and then we get accused of being condescending in our answers. 
 
What I would recommend is doing a little Chord Theory Study. It was the best thing I ever did in my musical life. Once you understand chord theory you can build any chord from scratch without having to look them up in a book. 
I don't read music but you don't need to read notation to study a lot of the music theory concepts. We can use the number system to describe a chord.
Example- a major chord is the I - III and V of the major scale.  We flatten the III to create a minor and so on. It's super easy once you "get it" 
 
A quick Google search finds lots of free sites with lessons. Example: 
 
http://www.pianoscales.org/chords.html
 
Bottom line is it certainly speed things up when I'm editing in Piano roll view and I'm editing chords. 
On complicated songs I'll insert markers on the time line with the chord names so I cane see what should be there when editing. 
 
Example if it says on the Time line the chord is a D, I know there should be D-F# and A. 
This is just a simple example as music gets a lot more complex than triads.