Create a new Java Project and name it XSWT.
Add the SWT jar files to the project's buildpath:
org.eclipse.swt.carbon.macosx.ppc_3.1.0.jar
org.eclipse.swt_3.1.0.jar
These are probably in /Applications/Eclipse/plugins
These instructions are for Eclipse 3.1 final. Details seem to change with each milestone.
Or just import org.eclipse.swt.*