To open a sheet you need to have two things, the window the sheet will be attached to and the window that's the sheet's contents.
To open the sheet:
modalForWindow:aWindow modalDelegate:aWindow didEndSelector:nil contextInfo:nil];
Where aSheet is the sheet you want to display and aWindow is the window it will be attached to.
To close the sheet:
Where aSheet is the sheet.
The usage of sheets is described in Mac OS X Human Interface Guidelines.