Only U.S. citizens can adopt a child in the U.S.  In case of married couples, at least one of the adopting parents needs to be a U.S. citizen.  Most importantly, the child needs to be legally available for adoption and this standard will vary from country to country. 

Links:

United States Citizenship and Immigration Services (USCIS)  
This site allows you to check on your case status with USCIS.
https://egov.uscis.gov/cris/jsps/index.jsp_

United States Department of State
This site allows you to see the most current visa bulletin.
http://travel.state.gov/visa/frvi/bulletin/bulletin_1360.html

United States Department of Labor
This site allows you to view up to date processing times and dates for all pending foreign labor certifications. 
http://www.foreignlaborcert.doleta.gov/

United States Customs and Border Patrol
www.cbp.gov

United States Immigration and Customs Enforcement
www.ice.gov

American Immigration Lawyer's Association (AILA)
www.aila.org