Some time ago I created a tutorial. You find it here: http://www.palmmedia.de/Blog/2012/9/27/cruisecontrol-net-integration-of-coverage-reports
The only mistake I can see in your configuration is, that you placed the quotes at the wrong position. Use
"-reports:C:\Program Files (x86)\OpenCover\coverage.xml"
instead of
-reports:"C:\Program Files (x86)\OpenCover\coverage.xml"