Every person who has ever lived on the earth will have the opportunity to accept or reject the Gospel of Jesus Christ. 
 
In that same vein anyone who as ever lived on the earth, has accepted the Gospel (either in this life or in the hereafter) will have the opportunity to have their proxy work done for them. One of the main activities during the Millenium (period of 1,000 years after Christ's 2nd coming) is doing all the genealogy and temple work. We believe that records are being kept in heaven similar to records we have on earth. Somehow (and I don't know how it will work) we will have access to information that is being recorded in heaven. Using that information proxy work can then be done in the temples for them. 
 
God is fair and is no respecter of persons - meaning God is not biased for / against anyone. Based on that assumption - then everyone will have the opportunity to learn about the gospel, accept or reject it, have ordinances performed for them, and be able to live eternally with Heavenly Father. 
 
Even though I might not know all the mechanics - I do know that everyone will have a fair chance. 
 
Hope that helps. 
 
Thanks 
 
Jett 
ps (These are solely my opinions and not necessarily those of the Church of Jesus Christ of Latter-Day Saints.)