recording voice with instrumental

All you need to do is mix your instrumental track and your vocals in such a way that you can hear both.
What you want to do is essentially mix your song. 
 
I would read up on allot mixing techniques as there are over 1,000,000,000,000,000 ways to mix a vocal track into a beat track.  Books are a good start and there is so much info on the internet to help you acquire a better understanding on what the art of mixing involves.
 
Also learn the tools that you will be using as the tools you use are only as good as the person adjusting them.
 
CJ

Sounds like an issue with exporting your track.
 
(1) Check that the output of your instrumental track is going to the master, the same as your new vocal track. 
 
(2) If there are options when exporting, beware of any checked boxs etc
 
(3) Does this only occur with mp3's? export as wav, do you get the same issue?
 
Cian

Here is a normal signal chain: Just think of Sonar, just like a real mixing board
 
All Tracks go to the master bus
Sends go to a specific bus of your choice
All buses go to the Master bus
Master bus goes to the Main outs 1/2