Under the compression options (I think) there is a screen which allows you to pick your MP3 encoding program. If you are using Lame then it will be set to User Defined. Under that there will be a box for switchs. This tells the Lame encoder how to compress your track. One of those switches is the one that says use vbr not cbr.
Unfortunatly I don't know which one it is. Going though help or probably to the website should tell you which switch to change.
Is there a readme file in the EAC directory on this. I think there might be.