Hi Bob,
See the updated documentation here: http://www.mojoportal.com/workingwiththesourcecodeinmonodevelop.aspx
The ISectionSettingsMapper is a new feature in mono that isn't in the 1.2.6 release but is in svn.
If using 1.2.6 you can get past this error by going into options on the Web project (right click choose options), under code generation remove USESETTINGSMAP compiler directive and it will leave out the code that is throwing the error.
Hope it helps,
Joe