1. Find the XML schema directory of your VS2010 Express (Beta 2, at this time) installation
Tools->Options will point you to the right place. Do not forget to select the Show all settings checkbox at the left bottom of the window:
2. Then, go to the folder you extracted the nhibernate binaries into. Go into the subfolder Required_Bins and copy the two .xsd files nhibernate-configuration.xsd and nhibernate-mapping.xsd
3. Copy them into the XMLSchemas folder you found in step 1