|   | 1 |  initial version  | 
My guess is that /opt/local/bin is not in your PATH for GUI applications.  If you open a terminal and start the sage server there, I bet it will work (and I think you indicate that it does).  
What happens is that GUI applications (including Sage.app) do not read .bash_profile etc. when they start up so they don't get any changes to PATH made there.  You have to edit the file ~/.launchd.conf (10.7+) or ~/.MacOSX/environment.plist and add the correct PATH there.  See this stackoverflow answer for details.
|   | 2 |  No.2 Revision  | 
My guess is that /opt/local/bin is not in your PATH for GUI applications.  If you open a terminal and start the sage server there, I bet it will work (and I think you indicate that it does).  
What happens is that GUI applications (including Sage.app) do not read .bash_profile etc. when they start up so they don't get any changes to PATH made there.  You have to edit the file ~/.launchd.conf (10.7+) or ~/.MacOSX/environment.plist and add the correct PATH there.  See this stackoverflow answer for details.
This is a very common problem on stackoverflow and similar sites. I wish I could figure out a way to make this easier for people to discover.
 Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.
 
                
                Copyright Sage, 2010. Some rights reserved under creative commons license. Content on this site is licensed under a Creative Commons Attribution Share Alike 3.0 license.