How to open existing project in Eclipse

Try File > New > Project... > Android Project From Existing Code. Don't copy your project from pc into workspace, copy it elsewhere and let the eclipse copy it into workspace by menu commands above and checking copy in existing workspace.

It's the "Import existing project into workspace" option under Import->General.

See: http://help.eclipse.org/helios/index.jsp?topic=%2Forg.eclipse.platform.doc.user%2Ftasks%2Ftasks-importproject.htm