If I am altering or doing an addition to my building, can I still follow the older 1991 ADA Standards for Accessible Design or must I use the new 2010 ADA Standards?

No, as of March 15, 2012, the applicable standards for alterations and additions are the 2010 ADA Standards for Accessible Design.   The 2010 ADA Standards can be found at: http://www.ada.gov/2010ADAstandards_index.htm.  

It is important to remember that the 1991 Standards are still relevant after March 15, 2012. Elements not altered after March 15, 2012 that comply with the requirements for those elements in the 1991 Standards, do not need to be modified, even if the new standards have different requirements for these elements.  This provision is called “Safe Harbor.”  However, if your business chooses to alter elements that were in compliance with the 1991 Standards, the safe harbor no longer applies to the altered elements which must now comply with the 2010 ADA Standards.